Class 9

West Bengal Board Class 9 Text Book Solutions WBBSE

• West Bengal Board Class 9 Maths Book Solutions
• West Bengal Board Class 9 Life Science Book Solutions
• West Bengal Board Class 9 Physical Science Book Solutions
• West Bengal Board Class 9 Geography Book Solutions
• West Bengal Board Class 9 History Book Solutions
• West Bengal Board Class 9 Bengali Book Solutions
• West Bengal Board Class 9 English Book Solutions
• West Bengal Board Class 9 Hindi Book Solutions
• WBBSE Class 9 Physical Science MCQ Questions
• WBBSE Class 9 Life Science MCQ Questions
• WBBSE Class 9 Geography MCQ Questions
• WBBSE Class 9 History MCQ Questions
• WBBSE Class 9 Physical Science Notes

WBBSE Solutions

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 2 Force and Motion offer valuable context and analysis.

WBBSE Class 9 Physical Science Chapter 2 Question Answer – Force and Motion

Very short answer-type questions

Question 1.
Of mass and weight of a body, which is more fundamental ?
Answer:
Mass is more fundamental than weight.

Question 2.
What is the ultimate form of all energies ?
Answer:
Heat energy.

Question 3.
In which part of the earth the weight of a body is maximum ?
Answer:
The weight of a body is maximum at the two poles of the earth.

Question 4.
Which type of balance is used to measure the weight of a body ?
Answer:
Spring balance.

Question 5.
What is the ultimate source of all energies ?
Answer:
The sun.

Question 6.
In an electric cell which type of energy changes to which other ?
Answer:
Chemical energy to electric energy.

Question 7.
What is the principle of solar cooker?
Answer:
In a solar cooker, heat energy of the sun is trapped in a box producing sufficient heat.

Question 8.
Why does wind possess energy ?
Answer:
Wind possesses energy due to speed of air particles.

Question 9.
How is tidal energy explored?
Answer:
Water stored at a higher level during high tide possesses potential energy that converts to kinetic energy when the water comes down during low tide.

Question 10.
What will be observed when the mass of a body is measured with a common balance on the surface of the earth and at the top of a mountain ?
Answer:
Mass of the body will be same at both the places.

Question 11.
When a body is taken to a high altitude from the earth, what changes of mass and weight of the body are observed ?
Answer:
Mass remains same, weight decreases.

Question 12.
Where does a body weigh more, at the equator or at the poles of the Earth ?
Answer:
A body weighs more at the poles than at the equator, for, the poles are nearer to the centre of the earth than the equator.

Question 13.
In what type of balance is mass of a body measured ?
Answer:
Mass of a body is measured in common balance.

Question 14.
Write down the relation between mass and weight.
Answer:
Weight of a body = mass of the body × acceleration due to gravity.

Question 15.
Why does the weight of a body change at different places ?
Answer:
Weight of a body given by the equation, w=m g, changes due to variation of ‘g’ at different places.

Question 16.
Does a spring balance show the same weight of a body at different places?
Answer:
No. Weight of a body changes at different places due to variation of ‘g’. So, a body weighed with a spring balance at different places show different weights.

Question 17.
Write down the names of non-conventional sources of energy.
Answer:
Some non-conventional sources of energy : (i) Solar energy (ii) wind energy (iii) Tidal energy.

Question 18.
Give an example of transformation of electrical energy to heat energy.
Answer:
In electric heater and electric iron, conversion of electrical energy into heat energy takes place.

Question 19.
Give an example of transformation of heat energy to electrical energy.
Answer:
Two different metal wires are joined at two ends. One end is kept in ice and the other end is heated. Electric current will flow through the wire. This is the conversion of heat energy into electrical energy.

Question 20.
Give an example of transformation of mechanical energy to electrical energy.
Answer:
In dynamo, rotation of a wire wheel in a magnetic field produces electric current. Hence mechanical energy of the wheel is converted into electrical energy.

Question 21.
Give an example of transformation of sound energy to electrical energy.
Answer:
In telephone and microphone, sound energy is converted into electrical energy.

Question 22.
Give an example of transformation of light energy to electrical energy.
Answer:
In photo-electric cell light produces electricity.

Question 23.
How is potential energy converted into kinetic energy ?
Answer:
When one object is kept at an elevated position, it is stationary there. Its total energy is potential energy. But when it is allowed to fall on the earth its potential energy is converted into kinetic energy.

Question 24.
Name an energy not derived from solar energy.
Answer:
Nuclear energy is not derived from solar energy.

Question 25.
Can energy be destroyed ?
Answer:
Energy can not be destroyed. It can only be transformed from one form to the other.

Question 26.
Is wood matter ?
Answer:
Wood is an example of matter as it exists in nature occupying some space.

Question 27.
Does mass of a body change with the change of place ?
Answer:
Mass of a body is its intrnsic property and it remains same everywhere.

Question 28.
What is the absolute unit of weight in SI system ?
Answer:
Newton.

Question 29.
What is the absolute unit of weight in CGS system ?
Answer:
Dyne.

Question 30.
What is the value of acceleration due to gravity (g) in SI system ?
Answer:
9.81 m / sec²

Question 31.
Give an example of transformation of kinetic energy to potential energy.
Answer:
When an object is thrown vertically upwards, its kinetic energy is converted into potential energy.

Question 32.
What will be the weight of a thing in artificial satellite?
Answer:
All things in satellite will be weightless.

Question 33.
Define the term displacement.
Answer:
The displacement of a moving particle in a given interval of time is the shortest distance between the two positions.

Question 34.
What do you mean by motion ?
Answer:
If a body changes its position with respect to time, the body is said to be in motion and that body is called kinetic body.

Question 35.
Define speed.
Answer:
Rate of change of position of a body with respect to time is known as its speed.

Question 36.
State SI unit of speed.
Answer:
SI Unit of speed is ms^-1.

Question 37.
Is velocity a scalar or a vector quantity ?
Answer:
Velocity is a vector quantity.

Question 38.
Define velocity.
Answer:
Rate of change of position of a body in particular direction with respect to time is called the velocity of the body.

Question 39.
Give one example of non-uniform velocity.
Answer:
When a train enters a station to stop, its velocity gradually decreases, while it leaves the station its velocity gradually increases.
In both the cases, velocity is unequal in equal time intervals i.e. the velocity of the train is non-uniform.

Question 40.
Define unbalanced forces.
Answer:
Unbalanced forces: When two forces of unequal magnitudes act in opposite directions on an object simultaneously, then the object moves inthe directions of the larger force. Such forces acting on the body are said to form a system of unbalanced forces.

Question 41.
State Newton’s first law of motion.
Answer:
Newton’s first law of motion : Every body continues in its state of rest or of uniform motion in a straight line unless it is acted on by some balanced force to change that state.

Question 42.
State Newton’s third law of motion.
Answer:
To every action there is an equal and opposite reaction.

Short answer type questions :

Question 1.
When a candle burns out, the molten wax weighs less than the candle. What happens to the ‘missing mass’?
Answer:
Explanation : The apparently missing mass of the candle converts to water vapour and carbon dioxide gas which disappears into air. The left out molten wax thus weighs less than the candle.

Question 2.
A thing is taken to Darjeeling from Kolkata. Is there be any change regarding its mass and weight?
Answer:
Explanation : Mass being intrinsic property of matter, it will remain same on both the places but weight being a force which is dependent on the value of ‘ g ‘ will be less at Darjeeling due to high altitude than that of Kolkata at sea-level.

Question 3.
Is there be any weight of a body falling freely on earth ? Give reasons.
Answer:
Force of attraction produces acceleration acting on a body falling freely towards the earth. In this condition acceleration due to gravity ‘ g ‘ will not act on the body. So, there will be no weight but the body will retain its mass because mass is the intrinsic property of matter.

Question 4.
Suppose the force of attraction of earth vanishes. What change would you see regarding mass and weight of a body?
Answer:
Explanation: The body will have its own mass but there will be no weight i.e. the body will be weightless.

Question 5.
Which is more fundamental, mass or weight ?
Answer:
Explanation : Mass is more fundamental than weight because mass remains unchanged at any place, whereas weight changes at different places on the Earth and on different planets.

Question 6.
What is the ultimate form of all energies ?
Answer:
Each form of energy ultimately transforms to heat energy directly or after some intermediate transformations.

Question 7.
Define matter.
Answer:
Matter : Matter is that which exists in nature occupying some space i.e. which have volume and mass and can be perceived by one or more of our senses. It exists in three states (a) solid (b) liquid (c) gas.

Question 8.
What do you mean by a body ?
Answer:
Body : Anything that is perceptible to our senses and has weight and free existence is called a body.
A body cannot change its state of motion of its own. A body is made up of matter.

Question 9.
What is energy ?
Answer:
Energy : Energy is that which exists in nature having no mass or does not occupy any space but can be perceived by our senses.

Question 10.
Define mass.
Answer: Mass : Mass is that intrinsic property of a body which determines the rate of change of state of motion in a straight line or of rest of the body when some external forces are applied on it.

Question 11.
What are the characteristics of mass ?
Answer:
Characteristics of mass :

1. It remains constant at any place of the universe, in any planet or any other place outside the universe.
2. It cannot be changed by the application of heat, by state of motion or rest, by electrification or magnetisation.
3. Mass of an object is its intrinsic property.

Question 12.
What is weight ?
Answer:
Weight : It is the force that a material body experiences towards the centre of the earth due to gravitational pull of the earth.

Question 13.
What is the relation between the mass and weight of a body ?
Answer:
Relation between the mass and weight of a body :
Weight of a body = mass of the body × acceleration due to gravity
If w be the weight of a body of mass ‘m’ and acceleration due to gravity be ‘g’ then, w = mg

Question 14.
What is acceleration due to gravity ?
Answer:
Acceleration due to gravity : The acceleration with which a freely falling body falls under the action of earth’s gravitational force is called acceleration due to gravity of the place and this is denoted by ‘g’.

Question 15.
What are the states of matter ?
Answer:
States of matter : There are three states of matter : solid, liquid and gas. Beside three states of matter there are two other states of matter, these are plasma state and super cooled liquid state.

Question 16.
Why can astronauts carry heavy loads on the moon but not on the earth ?
Answer:
The acceleration due to gravity on the moon is about onesixth than that on the earth. Astronauts can carry heavy loads easily on the surface of the moon, which is not possible on the earth.

Question 17.
State the law of conservation of mass.
Answer:
The law of conservation of mass (Lavoisier) : It states that the matter is neither created nor destroyed in a physical or chemical reaction i.e. the total mass of the components in a physical or chemical reaction remains unchanged after the reaction. The total mass of the universe is constant.

Question 18.
What do you mean by ‘energy’?
Answer:
Energy : Energy is defined as the capacity to perform work.
The amount of energy can be measured by the work done by the body. Its unit is same as the unit of work.

Question 19.
What is the law of conservation of energy ?
Answer:
Law of conservation of energy : The total amount of energy in this universe is constant. Energy cannot be created or destroyed. Energy can be transformed from one form to another.

Question 20.
What is solar pond technology ?
Answer:
Solar pond technology : In this technology, captured solar heat is kept at the bottom of a large expanse of saline water. Salty water layers are preventive to heat convection. Hot water from the bottom can be drawn whenever necessary and used for heating or forming electricity. In this way, solar ponds can form power regardless of whether the sun is out at the time.

Question 21.
What is Energy crisis ?
Answer:
Energy crisis : The limited stock of the common energy sources like coal, petroleum, natural gas cannot fulfill the demand of the huge requirements of energy for the large number of people and industry. But these sources are not infinitely large, they are going to be exhausted and cannot be renewed. These sources of non-renewable energy will be exhausted completely and the earth will face a great energy crisis.

Question 22.
What are non-conventional sources of energy ?
Answer:
Non-conventional sources of energy : To save the limited stock of coal, petroleum, scientists are in search of alternative sources of energy which are called non-conventional sources of energy. The main characteristic of these sources is that they are renewable. Some of these are (i) solar energy (ii) tidal energy.

Question 23.
State Einstein’s mass-energy equation.
Answer:
Einstein’s mass energy equation : The mass energy equivalence principle arises from Einstein’s famous theory of relativity.
Another form of energy is mass. Mass can be converted into energy and vice versa. If a mass ‘m’ can be converted completely into energy, then the amount of energy obtained from the equation.
E = mc²,(c = velocity of light in vacuum)

Question 24.
State the law of conservation of mass energy.
Answer:
Law of conservation of mass energy : In nature, the total amount of mass and energy is constant. Different types of transformation are possible between them. But mass and energy cannot be completely created or destroyed.

Question 25.
Discuss one simple example of conservation of mass in everyday life.
Answer:
Boiled rice has greater mass than that of the dry rice from which it is prepared. This happens because dry rice absorbs some water when boiled. So, total mass of dry rice and the absorbed water equals the mass of boiled rice.

Question 26.
Mention two differences between speed and velocity.
Answer:
Distinction between speed and velocity

Speed	Velocity
(i) Speed is the rate of change of position of a particle with respect to time.	(i) velocity is the rate of change of displacement of particle with respect to time.
(ii) Speed has only magnitude, so it is a scalar quantity.	(ii) Velocity has both magnitude and direction, so it is a vector quantity.

Question 27.
Define ‘newton’. Express it in CGS absolute unit.
Answer:
Newton : It is a force which when acting on a body of mass one kilogram produces in it an acceleration of one metre per second every second.
1 Newton = 1 kg × 1 ms²
= 1000 g × 100 cms²
= 10⁵ g.cms²
= 10⁵ dyne

Question 28.
What is frame of reference ?
Answer:
Frame of reference : It is any nearby or a distant fixed object from which the distance and position of a body is observed as time lapses.

Question 29.
What is meant by rest state of a body ?
Answer:
Rest state of body : A body is said to be at rest if it does not change its position with time from a fixed neighbouring object.

Question 30.
What is inertia of rest and what is inertia of motion ?
Answer:
Inertia of rest : The tendency of a body at rest to continue in that state is the inertia of rest.
Inertia of motion : The tendency of a body in uniform motion in a straight line to continue in that state is known as the inertia of motion.

Question 31.
State Newton’s second law of motion. What is dyne ?
Answer:
Newton’s second law of motion : When an unbalanced force acts on a body then the applied force is proportional to the acceleration produced in it.
Dyne : It is a force which when acting on an body of mass one gram produces in it an acceleration of one centimetre per second every second.

Broad answer-type questions :

Question 1.
Why does the value of ‘g’ vary from place to place?
Answer:
The value of ‘g’ varies from place to place :
(i) As we go up from earth’s surface or go down inside below the surface of the earth, the value of acceleration due to gravity decreases in both cases.
(ii) At the centre of the earth the value of acceleration due to gravity is found to be zero.
(iii) Moreover acceleration due to gravity changes due to earth’s spin about its axis also. Standard value of acceleration due to gravity is taken at 45° latitude at sea level.
(iv) Again on the surface of earth it is maximum at the pole. Because, due to orange shape of the earth, the distance of the pole is shorter than that of the equator from the centre of the earth.

Question 2.
Give an example of conservation of mass.
Answer:
Example : Burning of magnesium.
A small piece of magnesium ribbon is taken in the test tube and the stopper is tightly fitted. The test tube is filled with oxygen by replacing air in the test tube using the side tube and the delivery tube. The whole apparatus is weighed carefully. The test tube is now strongly heated on a bunsen burner. Magnesium ribbon burns brilliantly and after cooling a white residue remains in the test tube. The tube is again weighed. The weight of the test tube before and after heating is the same. The chemical reaction involves : 2 Mg + O₂ = 2 MgO. This proves the law of conservation of mass.

Question 3.
Write down the difference between mass and weight.
Answer:
Difference between mass and weight.

Mass	Weight
(i) The measure of inertia of a body in comparison to a standard body on the capacity of a body for gravitational attraction is the mass of the body.	(i) The force with which a body is attracted by the earth towards its centre is the weight of the body.
(ii) Mass of a body has only the magnitude, but no direction, so it is a scalar quantity.	(ii) Weight has both magnitude and direction and is therefore a vector quantity.
(iii) Mass of a body is its intrinsic property and it remains same everywhere.	(iii) Weight of a body is not its intrinsic property. The weight of a body depends upon the gravity of a place and it changes from place to place.
(iv) CGS and SI units of mass are gram (g) and kilogram(kg) respectively.	(iv) Absolute unit of weight in CGS system is Dyne and in SI system Newton. Gravitational units in CGS and SI are g-wt and kg-wt respectively.
(v) It is measured by an ordinary balance.	(v) It is measured by spring balance.
(vi) Mass of a body cannot be zero.	(vi) Weight of a body can be zero at the centre of the earth as ‘ g’ is zero there.

Question 4.
The sun is the most important source of energy on the earthExplain with Example.
Answer:
The sun is the most important source of energy on earth : After a few changes and transformations, almost all kinds of energy may be proved to originate from the sun.

Example : Let us Trace back the source of Hydel energy. The heat energy of solar radiation causes evaporation of water from different water bodies. This creates atmospheric water vapour. The water vapour forms cloud, rain, snow which are the sources of water for rivers. The river water is stored at an elevated position in dams. The water in dams possesses some kind of energy called potential energy. Falling water from the dams releases this energy forming another energy, known as kinetic energy. The kinetic energy of the falling water is used to rotate a turbine. The rotating turbine with the help of a generator produces hydel energy.

Question 5.
Discuss how wind energy and tidal energy may be utilised as sources of alternative energy.
Answer:
Wind energy : Wind mill or wind generator is used to produce electricity from air which has speed of at least 18 to 20 km / hr. The kinetic energy of wind is converted into the mechanical energy of the fans of the wind mill, which in turn runs the generator and produces electricity. Wind is the most pollution free energy for this reason. It is the world’s most rapidly developing new source of energy.

Tidal energy : In some coastal areas, the difference in water level between high tide and low tide is as high as 6 to 8 m. A portion of a river in such a region is blocked so that water in this part of river stores during high tide. This stored water gets potential energy. During low tide, the stored water is allowed to flow to lower level when the potential energy is transformed into kinetic energy which rotates a water turbine set on the path of water flow. The rotating turbine produces electric energy.

Question 6.
‘With the help of a common balance we measure the comparative mass of a body but not its weight’-Justify the statement.
Answer:
In a common balance, a body is counterpoised with standard units of mass, so mass of the body is comparatively measured. Acceleration due to gravity ‘ g ‘ acts equally on both the masses, so both the masses are equally pulled downwards by the earth. If the mass of the body is denoted as m and the standard mass be M, then we can write m g=M g that is m=M. Thus, in a common balance, mass of the body is determined in comparison with that of the standard weights. Weight of the body is not obtained in this process and that is why difference of weight of a body cannot be detected at different places with a common balance.

Question 7.
Give some examples of transformation of energy.
Answer:
Some examples of transformation of energy :

Question 8.
Deduce Newton’s first law of motion from the second law.
Answer:
From the relation, force = mass × acceleration (or retardation) if the applied force be zero i.e. in absence of any external force, acceleration or retardation is zero, since, mass cannot be zero. Now, acceleration or retardation is zero means, either the body is at rest or it is in uniform motion ; thus, in absence of an external force, a body is either at rest or in uniform motion, that is Newton’s first law of motion.

Question 9.
Why is it easier to accelerate a moving vehicle by push than one at rest ?
Answer:
Reason : When a car or a roller is at rest, to set it into motion, a large force is necessary to overcome the inertia of rest of its large mass and also to overcome its frictional force with ground. On the other hand, when it is already in motion, Since it itself tries to maintain its forward motion due to inertia of motion, a smaller effort can accelerate it.

Question 10.
How is the definition of force obtained from Newton’s first law of motion ?
Answer:
Newton’s first law of motion states that in absence of an external force the state of rest or of uniform motion of a body in a straight line does not change. Hence, force is an agent which acting on a body, either actually changes or tends to change the state of rest or uniform motion of the body in a straight line. This is the definition of force derived from Newton’s first law of motion.

Question 11.
If a ball is thrown straight upward by a passenger in a train running with uniform velocity it returns to his hand. Explain.
Answer:
Explanation : A ball thrown straight upward by a passenger in a train running with uniform velocity, returns to his hand although he has moved forward with the train. The ball which is also moving with the speed of the train, maintains its motion in the horizontal direction due to inertia of motion and returns to the hand.

Question 12.
What are the points in connection with action and reaction to be noted carefully ?
Answer:
The following points in connection with action and reaction are to be noted carefully.

1. Reaction exists as long as the action exists. In absence of action there will be no reaction.
2. Action and reaction between two bodies act along the straight line joining the bodies.
3. In nature, forces always exist in pairs. They never exist singly. A single isolated force is impossible.
4. Newton’s third law is applicable between two bodies irrespective of whether they are in motion or at rest, in contact or not.

Numerical Problems

Working formula:

(i) Weight of the body = mass of the body × acceleration due to gravity
= m × g
(ii) Value of ‘g’ on the surface of the moon
= \(\frac{1}{6}\) × value of ‘g’ on the earth
(iii) Average value of ‘ g ‘ on the earth’s surface in CGS system = 981 cm / sec² and in SI system = 9.81 m / sec²
(iv) Momentum of a moving body = mass of the body (m) × velocity of the body (v)
(v) (a) p = \(\frac{(m v-m u)}{t}\)
(b) p = m f.
[where p = force applied, m = mass of the body, u = initial velocity.
v = final velocity, t = time, f = acceleration]
(vi) Law of conservation of momentum :
m₁ u₁ + m₂ u₂ = m₁ v₁ + m₂ v₂

Example 1 : The reading in a spring balance of a body of mass 20 kg is found to be 196 newton. What is the value of acceleration due to gravity of the place?
Answer:
We know, w = m × g
or, g = \(\frac{w}{m}\)
or, g = \(\frac{196}{20}\) = 9.8 m / s²
w = weight of the body
= 196 N
m = mass of the body
= 20 kg
g = acceleration due to
gravity = ?
So, the acceleration due to gravity = 9.8 m / s²

Example 2 : What will be the weight of a body of mass 200 g at a place where g = 980 cm / sec 2.
Answer:
We know,
w = m × g
∴ w = 200 × 980 g . cm / sec²
= 196000 dyne

So, the weight of the body = 196000 dyne

w = weight of the
body = ?
m = mass of the body
= 200 g
g = acceleration due
to gravity
= 980 cm / sec²

Example 3: What will be the weight of a body on the surface of the moon which weighs 240 kg on the earth?
Answer:
We know,
Value of ‘ g ‘ on the surface of the moon = \(\frac{1}{6}\) × value of ‘ g ‘ on the earth
So, the weight of the body on moon = \(\frac{1}{6}\) × 240 kg = 40 kg

Example 4 : A body has mass 20 kg. What will be the weight of the body ? (g = 9.81 m / sec 2)
Answer:
We know, w = m × g
or, w = 20 × 9.81 kg-m / sec²
∴ w = 196.2 Newton
So, the weight of the body

w = weight of the
body = ?
m = mass of the body = 20 kg
g = 9.81 m / sec²
= 196 . 2 newton.

Example 5 : A body has mass 30 kg on the earth. What will be the mass and weight of the body in artificial satellite of the earth ?
Answer:
In artificial satellite of the earth the mass of the body will be same i.e. the mass of the body will be 30 kg.
The weight of the body will be zero in artificial satellite of the earth.

Example 6 : What will be the magnitude of the force acting on a mass of 10 kg to produce an acceleration of 4 m / s 2 ?
Answer:
We know,
p = m × f
∴ p = 10 × 4 = 40 Newton
m = 10 kg
f = 4 m / s²
p = ?
∴ The force required is 40 Newton

Example 7 : Calculate the acceleration of a body of 250 g when acted upon by a force of Q 1 Newton,
Answer:
We know, p = m × f
Or, f = \(\frac{p}{m}\) = \(\frac{1}{0.25}\) = 4 m / s²
p = 1 N
m = 250 g
= 0.25 kg
f = ?

Example 8: A force is acting on a mass of 20 g. The initial velocity of the particle is 10 cm / s. After 5 s, the velocity is increased to 40 cm / s. Find the magnitude of the force.
Answer:
We know,
v = u + f t
∴ f t = v – u
or, f = \(\frac{40-10}{5}\) = 6 cm / s²
We also know,
p = m × f
= 20 × 6 = 120 dyne.
u = 10 cm / s
m = 20 g
v = 40 cm / s
t = 5 s
f = ?
m = 20 g
f = 6 cm / s²
p = ?

Example 9 : A force of a 4 kg-wt acts on a body of mass 9.8 kg.
Calculate the acceleration.
(Take g = 9.8 m / s² )
Answer:
We know,
4 kg-wt = 4 kg × 9.8 m / s² = 39.2 N
Now, force = mass × acceleration
∴ acceleration = \(\frac{\text { force }}{\text { mass }}=\frac{39.2 \mathrm{~N}}{9.8 \mathrm{~kg}}\)
= 4m / s²

Example 10 : What will be the acceleration of a body of mass 15 kg when a force of 45 newton is applied on it ?
Calculate the acceleration.
Answer:
We know,
p = m / f
m = 15 kg
p = 45 N
f = ?
or,
f = \(\frac{p}{m}\)
= \(\frac{45}{15}\) = 3m / s²

Example 11 : A car is initially moving with an acceleration of 1.8 m / s². After some time the driver accelerates the car and it moves with an acceleration of 2.7 m / s². What is the ratio of the force exerted by the engine in the two cases?
Answer:
In the first case,
the force,
p₁ = mass × acceleration
= m × 1.8 N(m = mass of the car )
In the second case, the force
p₂ = mass × acceleration
= m × 2.7 N
∴ \(\frac{p_1}{p_2}=\frac{m \times 1.8}{m \times 2.7}=\frac{2}{3}\)

Example 12 : Equal forces act on two masses m and 2 m. If the acceleration acquired by m be f, what is the acceleration acquired by 2 m ?
Answer:
From the relation, force = mass × acceleration, the force acting on the mass m is m f.
Now the same force m f acts on the mass 2 m, and since acceleration = \(\frac{\text { force }}{\text { mass }}\) , the acceleration of the second body = \(\frac{m f}{2 m}=\frac{f}{2}\)

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 6 Heat offer valuable context and analysis.

WBBSE Class 9 Physical Science Chapter 6 Question Answer – Heat

Very short answer type questions

Question 1.
What is the unit of heat in CGS system ?
Answer:
Calorie is the unit of heat in CGS system.

Question 2.
What is the unit of heat in SI system ?
Answer:
In SI system, the unit of heat is Joule.

Question 3.
What is steam point?
Answer:
Steam point is the boiling point of water under normal pressure.

Question 4.
What is 1 calorie?
Answer:
1 calorie is defined as the heat required to raise the temperature of lg of water through one degree Celsius.

Question 5.
What is the unit of specific heat in CGS system ?
Answer:
The unit of specific heat in CGS system is cal/g/°C

Question 6.
What is the unit of specific heat in SI system ?
Answer:
The unit of specific heat in SI system is J/kg/K.

Question 7.
What is fundamental interval ?
Answer:
It is the range of temperature between the upper and lower fixed points in the scale of temperature.

Question 8.
What are the scales of temperature ?
Answer:
There are three scales of temperature —

• Celsius scale
• Fahrenheit scale &
• Kelvin scale.

Question 9.
What is the value of 100°C in Fahrenheit scale ?
Answer:
In Fahrenheit scale the temperature is 212°F.

Question 10.
What are the factors for determining the quantities of heat in a body ?
Answer:
The quantity of heat depends upon the following factors :

• temperature
• mass and
• material of the body.

Question 11.
Is heat a vector quantity ?
Answer:
No, heat is a scalar quantity.

Question 12.
What is the relation between calorie and joule ?
Answer:
1 calorie = 4.18 joule

Question 13.
What is the principle of calorimetry ?
Answer:
The principle of calorimetry is :
Heat absorbed by the cold body
= Heat given out by the hot body.

Question 14.
Is there any change of temperature during the change of state ?
Answer:
There is no change of temperature during the change of state.

Question 15.
Which temperature is lower — 0°C or 0°F ?
Answer:
0°F is lower than 0°C.

Question 16.
What are the lower fixed point and upper fixed point of thermometer in Celsius scale ?
Answer:
Lower fixed point is 0°C and upper fixed point is 100°C.

Question 17.
What are the lower fixed point and upper fixed point of thermometer in Fahrenheit scale ?
Answer:
Lower fixed point is 32°F and upper fixed point is 212°F.

Question 18.
What is the fundamental interval of Celsius scale ?
Answer:
The fundamental interval is divided in 100 equal parts in Celsius scale.

Question 19.
What is the fundamental interval of Fahrenheit scale ?
Answer:
The fundamental interval is divided into 180 equal parts in Fahrenheit scale

Question 20.
What is the melting point of ice in Kelvin scale ?
Answer:
Melting point of ice in Kelvin scale is 273K.

Question 21.
What type of scale is used in clinical thermometer ?
Answer:
Generally Fahrenheit scale is used in clinical thermometer.

Question 22.
Which temperature is same in both Celsius scale and Fahrenheit scale ?
Answer:
– 40° temperature will read same both in Celsius scale and Fahrenheit scale.

Question 23.
Which substance has the highest specific heat ?
Answer:
Water has the highest specific heat.

Question 24.
What is specific heat capacity ?
Answer:
The specific heat in SI system is called specific heat capacity.

Question 25.
What is the value of specific heat capacity of pure water ?
Answer:
The specific heat capacity of pure water is 4200 J/kg/K.

Question 26.
Is specific heat of solid greater than that of liquid ?
Answer:
Generally specific heat of solid has lower value than that of liquid.

Question 27.
What is the normal temperature of human body ?
Answer:
The normal temperature of human body is 98-4°F or 36-9°C.

Question 28.
If a substance has thermal capacity 100 calorie, what will be its water equivalent ?
Answer:
If the substance has thermal capacity 100 calorie, then water equivalent of the substance will be 100 g.

Question 29.
What is the relation between Celsius scale and Kelvin scale ?
Answer:
If a temperature in Kelvin scale is T K and the temperature in Celsius scale is t°C, the relation is :
TK = 273 + t°C

Question 30.
Give an advantage of use of mercury in thermometer.
Answer:
Equal volume of mercury increases or decreases for each degree increase or decrease of temperature in a regular manner.

Question 31.
The specific heat of copper is 0.09. What does the statement mean ?
Answer:
‘Specific heat of copper is 0-09’ means that 0 09 calorie of heat is required to raise the temperature of lg of copper through 1°C.

Question 32.
What is the value of 30°C in Kelvin scale ?
Answer:
The given temperature of is 303 K in Kelvin scale.

Question 33.
What is the boiling point of water in Kelvin scale ?
Answer:
The boiling point of water in Kelvin scale is 373 K.

Question 34.
By which process, heat reaches the earth from the sun ?
Answer:
Radiation.

Question 35.
What is heat ?
Answer:
It is an external agency whose absorption turns a body hot and extraction turns a body cold.

Question 36.
What is temperature ?
Answer:
The thermal condition of a body which determines whether heat will flow from the body to another body in contact or not is called the temperature of the body.

Question 37.
Define thermal capacity.
Answer:
Thermal capacity of a body is the quantity of heat required to raise the temperature of the body through one degree.

Question 38.
What is the unit of thermal capacity ?
Answer:
In CGS system the unit of thermal capacity is calorie per degree Celsius and in SI system it is joule per degree kelvin.

Question 39.
What are the value of latent heat of fusion of ice in SI and CGS system ?
Answer:
The values of latent heat of fusion of ice in SI system and CGS systems are 3.36 × 10⁵ Jkg^-1 and 80 cal g^-1 respectively.

Question 40.
Does saturated vapour obey Boyle’s law ?
Answer:
Saturated vapour does not obey Boyle’s law.

Question 41.
Does unsaturated vapour obey pressure law ?
Answer:
Unsaturated vapour obeys pressure law.

Question 42.
At what temperature is the density of pure water become maximum ?
Answer:
4°C

Question 43.
Can a certain quantity of unsaturated vapour be made saturated ?
Answer:
A certain quantity of unsaturated vapour can become saturated if the pressure be increased or the temperature be decreased at constant volume.

Question 44.
State two favourable conditions for formation of dew.
Answer:
The favourable conditions for the formation of dew are –

• Clear sky and
• still air

Short answer type questions

Question 1.
What is heat ?
Answer:
Heat : It is an external agency whose absorption turns a body hot and extraction turns a body cold.

Question 2.
What are the types of heat ?
Answer:
Types of heat : There are three types of heat. These are :

• sensible heat
• latent heat
• radiant heat

Question 3.
What is sensible heat ?
Answer:
Sensible heat: It is the type of heat that is perceptible by senses. This causes rise of temperature of a body that can be measured with a thermometer.

Question 4.
What is latent heat ?
Answer:
Latent heat: It is the quantity of heat absorbed or released when a substance of unit mass undergoes a physical change of state at a constant temperature and pressure. This heat is not detectable with a thermometer.

Question 5.
What is radiant heat ?
Answer:
Radiant heat : It transmits from a source of heat in all directions with the speed of light and in the form of waves.

Question 6.
What is temperature ?
Answer:
Temperature : The temperature of a body is that thermal condition of the body which determines whether it will absorb heat from other body or release heat to that body when they are kept in close contact.

Question 7.
What is lower fixed point of a thermometer ?
Answer:
Lower fixed point of a thermometer : At normal atmospheric pressure, the temperature at which pure ice melts into water or pure water freezes into ice is called the lower fixed point of a thermometer.

Question 8.
What is upper fixed point of a thermometer ?
Answer:
Upper fixed point of a thermometer : It is the temperature at which pure water boils and transforms into steam under normal atmospheric pressure.

Question 9.
What do you mean by fundamental interval ?
Answer:
Fundamental interval : The range of temperature between the upper and the lower fixed points is scalled Fundamental interval.

Question 10.
What do you mean by Celsius scale of temperature ?
Answer:
Celsius scale of temperature : In this scale the lower fixed point is 0°C and the upper fixed point is 100°C. The fundamental difference is divided into hundred equal parts and each parts represents 1° Celsius.

Question 11.
What do you mean by Fahrenheit scale of temperature ?
Answer:
Fahrenheit scale of temperature : In this scale the lower fixed point is 32°F and the upper fixed point is 212°F. The fundamental difference is divided into 180 equal parts and each part represents 1°F.

Question 12.
What is absolute scale or Kelvin scale of temperature ?
Answer:
Absolute or Kelvin scale of temperature : In this scale the lower fixed point is 273K and the upper fixed point is 373K. The fundamental interval is divided into 100 equal divisions like that in the Celsius scale. Each division represents 1K.

Question 13.
What are the units of heat in CGS and SI system ?
Answer:

• Unit of heat in CGs system is calorie : It is the amount of heat required to raise the temperature of one gram of pure water through 1°C.
• Unit of heat in SI system is joule : As heat is a form of energy so nowadays heat is measured in the unit of energy. That is heat is expressed in joule. It is the SI unit of heat.
  1 calorie = 4.18 joule.

Question 14.
What do you mean by specific heat of a substance ?
Answer:
Specific heat of a substance : It is defined as the quantity of heat required to raise the temperature of unit mass of it by 1 degree.

Question 15.
What is specific heat capacity of a substance ?
Answer:
Specific heat capacity of a subsance : The specific heat in SI system is known as specific heat capacity.
Definition : Specific heat capacity of a substance is the quantity of heat required to raise the temperature of 1 kilogram of the substance through 1K.

Question 16.
What are the units of specific heat in CGS and SI system ?
Answer:
Unit of specific heat in CGS system is : cal/g/°C
Unit of specific heat in SI system is : J/kg/K

Question 17.
‘The specific heat of copper is 0.09’ – What does the statement mean ?
Answer:
The statement that ‘the specific heat of copper is 0-09’ means that 0.09 units of heat (calorie) is required to raise the temperautre of unit mass (1g) of copper through one degree (1°C)

Question 18.
What is thermal capacity or heat capacity of a body ?
Answer:
Thermal capacity or heat capacity of a body : It is defined as the quantity of heat needed to raise the temperature of the body for unit degree rise of temperature.

Question 19.
What are the units of heat capacity in CGS system and SI system ?
Answer:
Unit of heat capacity in CGS system is : Calorie/°C.
Unit of heat capacity in SI system is : Joule/kelvin

Question 20.
Deduce the relation between thermal capacity and specific heat of a body
Answer:
Relation between thermal capacity and specific heat of a body :
Quantity of heat required to raise the temperature of a body of mass
‘m’ and specific heat s by 1° = m.s.1 = m.s
i.e. Thermal capacity = ms
or, Thermal capacity = mass x specific heat

Question 21.
Show that specific heat is the heat capacity of unit mass.
Answer:
Thermal capacity of a body
= mass x specific heat of material of the body
Hence, thermal capacity = ms
Putting, m = 1, in the above relation,
Thermal capacity Specific heat
So, it can be concluded, specific heat is the thermal capacity per unit mass.

Question 22.
What is water equivalent ?
Answer:
Water equivalent: The water equivalent of a body is the mass of water which will be heated through one degree by the amount of heat required to raise the temperature of the body through one degree.

Question 23.
What are the units of water equivalent in CGS and SI system ?
Answer:
Unit of water equivalent in CGS system is : gram (g).
Unit of water equivalent in SI system is : Kilogram (kg)

Question 24.
What are the differences of thermal capacity and water equivalent ?
Answer:
Differences of thermal capacity and water equivalent

Thermal capacity	Water equivalent
(i) Thermal capacity represents some quantity of heat. (ii) Units of thermal capacity in CGS and SI system are calorie/°C and J/K respectively.	(i) Water equivalent repre­sents some quantity of water. (ii) Units of water equivalent in CGS and SI system are gram and kg respectively.

Question 25.
What is the fundamental principle of calorimetry ?
Answer:
Fundamental principle of calorimetry :
If no heat flows to outside from the system of bodies and if no chemical reaction takes place between them, then from the principle of conservation of energy, it can be said,
heat lost by the hotter body = heat gained by the colder body.
This is the principle of calorimetry.

Question 26.
What is the relation between heat and molecular velocity ?
Answer:
Relation between heat and molecular velocity : The molecules in a hotter body have more rapid motion and those of a less hot body have slower motions. Also, if heat is supplied to a body, its molecules move more rapidly. If heat is withdrawn from a body the motion of its molecules becomes slower. Thus, increase or decrease of melecular velocity corresponds to higher or lower quantity of heat.

Question 27.
Why should the bore of a thermometer be uniform ?
Answer:
Reason: The bore should be uniform, otherwise, unequal expansion of mercury thread will take place at different parts of the bore for equal change of temperature.

Question 28.
What is meant by sensitivity of a thermometer ?
Answer:
Sensitivity of a thermometer : A thermometer is considered to be sensitive if it :

• quickly picks up the temperature of the body with which it is kept in contact
• can detect small changes of temperature.

Question 29.
Why should the bore of a thermometer be narrow ?
Answer:
Reason : The bore of thermometer tube should be narrow so that expansion of mercury thread is long enough for a small change of temperature.

Question 30.
Why is water preferred than other liquids in a hot water bag ?
Answer:
Reason: Since water has the highest specific heat, a certain mass of water heated to a certain temperature contains more heat than the same mass of any other liquid heated to the same temperature. So, hot water will take longer time to cool than any other liquid heated to the same temperature. So, for hot compression it can be used for sufficiently long time.

Question 31.
What does the first law of thermodynamics state ?
Answer:
First law of thermodynamics :
When work is completely converted into heat or heat is completely converted into work, one is equivalent to the other.
Mathematical expression of first law of thermodynamics :
If by converting work W completely heat H is obtained then W α H
or W = JH (J = mechanical equivalent of heat)

Question 32.
What is mechanical equivalent of heat ? What are the units of mechanical equivalent of heat in CGS and Si system ?
Answer:
Mechanical equivalent of heat : Work done to produce unit amount of heat is called mechanical equivalent of heat.
Unit of mechanical equivalent of heat :
CGS system: 4 18 x 10⁷ erg/cal
SI system : 4 18 J/Cal

Broad answer type questions

Question 1.
What are the differences between heat and temperature ?
Answer:
Difference between Heat and Temperature :

Heat	Temperature
(i) Heat is a form of energy.	(i) Temperature indicates the thermal condition or degree of hotness or coldness of a body.
(ii) When a body absorbs heat, its temperature, in general, rises i.e. heat is the cause of temperature.	(ii) Temperature is the effect of heat.
(iii) Equal quantities of heat in two bodies do not signify equal temperature.	(iii) Equal temperature of two bodies does not signify equal heat in them.
(iv) During change of state of matter, total amount of heat alters.	(iv) During change of state, temperature remains unaltered.

Question 2.
What are the advantages of mercury as a thermometric substance?
Answer:
Advantages of mercury as a thermometric substance :

• Mercury being a metal, it absorbs very little heat from the source and thus there is almost no change in temperature of the source.
• Mercury is a shinning liquid, so can be easily seen through glass.
• Mercury remains in liquid state for long range of temperature (-39°C to 357°C). So temperature can be measured to a long range.
• Mercury does not wet the glass wall of tube, so it can move up and down easily through the glass tube.

Question 3.
What steps should be taken to make a thermometer sensitive ?
Answer:
Requisites of a sensitive thermometer :

• The thermometer bulb should have thin walls, so that heat from
  a body in its contact may easily reach the mercury inside it.
• The bore of thermometer-tube should be narrow so that expansion of mercury thread is long enough for a small change of temperature.
• The bore should be uniform, otherwise, unequal expansion of mercury thread will take place at different parts of the bore for equal change of temperature.
• The thermometric liquid should be good conducting so that it quickly assumes the temperature of the body in contact of the thermometer.

Question 4.
Obtain an expression for the heat absorbed or given out by a body.
Answer:
Calculation of amount of heat absorbed or given out by a body :
If S be the specific heat of substance, then we can write from the definition of specific heat that,
1 unit mass of a substance for 1° rise or fall of temperature will absorb or give out ‘s’ units of heat.
∴ m unit mass of a substance for t° rise or fall of temperature will absorb or give out ‘mst’ units of heat. That is,
Quantity of heat (absorbed or given out)
= mass of the body x specific heat × rise or fall of temperature.

Question 5.
What is the relation between Celsius, Fahrenheit and Kelvin scales ?
Answer:
Relation between Celsius, Fahrenheit and Kelvin scales:
Since the range of temperature from lower fixed point to upper fixed point is equal in all three scales, 100 centigrade degrees = (212 – 32) or 180 Fahrenheit degree = (373 – 273) or 100 absolute degrees.
We consider three thermometers in the above three scales are dipped simultaneously in a liquid of certain temperature. Let the temperatures recorded in the Celsius, Fahrenheit and Kelvin thermomters respectively be C, F, and K. Now it can be proved that

Question 6.
Equal masses of milk and water are separately taken in two identical vessels. The vessels are heated simultaneously on the same oven. Which one will have quicker rise of temperature — milk or water and why ?
Answer:
Explanation: Specific heat of milk is less than that of water, so milk will require a smaller quantity of heat for each degree rise of temperature that needed for the same mass of water. So, supply of heat being same to the equal masses of milk and water, rise of temperature of milk will be quicker than that of water.

Numerical problems

Working formula :

\(\frac{C}{5}=\frac{F-32}{9}=\frac{K-273}{5}\)
(Let the temperatures recorded in the Celsius, Fahrenheit and Kelvin thermometers respectively be C, F and K)

(ii) \(1^{\circ} \mathrm{C}=\frac{9^{\circ}}{5} \mathrm{~F} ; 1^{\circ} \mathrm{F}=\frac{5^{\circ}}{9} \mathrm{C}\)

(iii) Relation between temperatures of any two scales, X and Y :
\(\frac{\text { Reading in } X-\text { Lower fixed point of } X}{\text { Fundamental interval of } X}\)
\(=\frac{\text { Reading in } Y-\text { Lower fixed point of } Y}{\text { Fundamental interval of } Y}\)

(iv) Thermal capacity = ms (m = mass of the body ; s = specific heat)

(v) Water equivalent = ms

(vi) Quantity of heat (absorbed or given out) = ms (t₂ – t₁)
(m – mass ; s = specific heat ;
t₁ and t₂ are two different temperature)

(vii) Principle of calorimetry :
Heat lost by the hotter body = Heat gained by the colder body

(viii) W = JH (W = work done, H = heat produced, J = mechanical equivalent of heat)

Problem 1.
The temperature on a day was 94°F. What is it in celsius scale?
Answer:
We Know that

Problem 2.
The lowest attainable temperature so far is – 270° celsius. What is it in Fahrenheit scale?
Answer:
C = -270°
F = ?
We know,

or, F – 32 = – 54 x 9 = – 486
or, F= – 486 + 32 – 454
∴ F= – 454°

Problem 3.
Convert 50°C in Fahrenheit scale.
Answer:
C= 50°
We know,

or, F – 32 = 10
or, 90F – 32 × 9
or, F= 90 + 32 = 122
Hence, 50°C = 122°F.

Problem 4.
The rise in temperature of a body is 600 in celsius scale. What is the corresponding rise in temperature in Fahrenheit scale?
Answer:
We know, 1°C = \(\frac{9}{5}\)°F
∴ 60°C = (\(\frac{9}{5}\) x 60)°F = 108°F

Problem 5.
The difference in the readings of a Fahrenheit and a Celsius thermometer is 480. What are the actual readings shown in the thermometers ?
Answer:
Let the actual readings in the Fahrenheit and the Celsius thermometers correspond to F and C.
∴ F – C= 48° or, F = C+ 48°
Again, from the relation,

or, 9C = 5C + 80 or, 4C 80 or, C = 20
Now, F = C + 48° = 20° + 48° = 68°

Problem 6.
A certain temperature is 104°F, what is its value in Kelvin scale ?
Answer:
We know the relation between Fahrenheit and Kelvin scale is

Problem 7.
A thermometer having the corresponding upper and lower fixed points 95° and 15°, reads a temperature 65°. What will this temperature be in Celsius scale ?
Answer:
The relation between temperature of celsius and the given scale can be written from the general relation between any two scales as,

∴ The required temperature is 625°C.

Problem 8.
The normal temperature of human body is 98 4°F. What is the value of this temperature in centigrade scale ?
Answer:
We know,

So the required temperature = 3689°C.

Problem 9.
How much calorie of heat would be produced by converting 42 Joule of work completely into heat?
Answer:
We know, W = JH
or, \(\mathrm{H}=\frac{W}{J}=\frac{42 \mathrm{Joule}}{4 \cdot 2 \mathrm{Joule} / \mathrm{cal}}=10 \mathrm{cal}\)

Problem 10.
Which temperature is same in both Centigrade scale and Fahrenheit scale ?
Answer:
Let the temperature be x°
According to the question, C = F = x
We know,

or, x = \(\frac{-160}{4}\) = – 40
So, – 40° temperature will read same both in Fahrenheit and Centigrade scale.

Problem 11.
Mass and specific heat of a substance is 100g and 0.09 respectively. Find the thermal capacity and water equivalent of
the body.
Answer:
We know,
m = 100g; s = 0.09
Thermal capacity = ms
= 100 × 0.09 cal/°C
Water equivalent = ms
= 100 × 0.09 = 9g

Problem 12.
Calculate the amount of heat required to raise the temperature of 60g of water from 20°C to the boiling point.
Answer:
m = 50g
t₁ = 20°C
t₂ = 100°C
s = 1
We know,
Absorbed heat = ms (t₂ – t₁)
50 × 1 × (100 – 20)
= 50 × 80
= 4000 cal

Problem 13.
A body of iron having mass 60 g is cooled from 200°C to 100°C. Calculate the amount of heat given out. Specific heat of iron = 0.12.
Answer:
m = 60g
t₁ = 100°C
t₂ = 200°C
S = 0.12
We know,
the amount of heat given out by iron =
= ms (t₂ – t₁)
= 60 × 0.12 (200 –  100)
= 60 × 0.12 × 100 = 720 cal

Problem 14.
300 g lead at 99°C is dropped in 100 g water at 18°C. The final temperature of the mixture is 27°C. Find the specific heat capacity of lead if specific heat capacity of water is 4200 J kg-1 K-1.
Answer:
Let the specific heat capacity of lead be ‘s’ J/kg/K.
Heat lost by 300 g lead to reach the temperature to 27°C
= \(\frac{300}{1000}\) = × s × (99 – 27) J
= 0.3 × s × 72 Joules = 21.6s J
Heat gained by 100 g water when the temperature rises from 18°C to 27°C
= 100 × 42000 × (27 – 18) J = 3780 J.
So, from the principle of calorinetry, heat lost heat gained
∴  21.6s = 3780
or, s = \(\frac{3780}{21 \cdot 6}\) 175 JKg-1 K-1

Problem 15.
A piece of copper heated to 35°C is dropped into 140 g of water at 15°C. The mass of copper is 150 g and the specific heat of copper is 400 J/kg/K. Find the final temperature. (Specific heat of water = 4200 J/kg/K)
Answer:
Let the final temperature be t°C.
Now, the heat lost by copper.
= 0.15× 400 × (35 – t) = 60 (35 – t)J
Heat gained by water
= 0.14 × 4200 × (t – 15) = 588 (t – 15) J
So, from the principle of calorimetry,
heat lost = heat gained
∴ 60 (35 – t) = 588 (t -15)
or, 60 × 35 – 60t = 588 t – 588 × 15
or, 60 × 35 + 588 × 15 = 588t + 60t
∴ 648t = 60 x 35 + 588 x 15
or, t =\(\frac{60 \times 35+588 \times 15}{648}\) = 16.85°C

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 7 Sound offer valuable context and analysis.

WBBSE Class 9 Physical Science Chapter 7 Question Answer – Sound

Very short answer type questions

Question 1.
How is the sound produced when we speak ?
Answer:
Sound is produced due to vibration of our vocal chord.

Question 2.
How does the velocity of sound depend on pressure ?
Answer:
Velocity of sound is independent of change of pressure.

Question 3.
What is musical sound ?
Answer:
It is a pleasing sound produced by a periodically vibrating body.

Question 4.
How does intensity of sound depend on density of medium ?
Answer:
Intensity of sound increases in a denser medium than in a rarer medium.

Question 5.
What is note of sound ?
Answer:
It is a sound of mixed frequencies.

Question 6.
What are the characteristics of a musical sound ?
Answer:
Characteristics of a musical sound are, it has

• Intensity
• Pitch
• Quality or Timbre

Question 7.
Which characteristic of musical sound depends on frequency ?
Answer:
Pitch depends on frequency.

Question 8.
Find the optical analogous of note and tone,
Answer:
Note is analogous to white or polychromatic light and tone is analogous to light of a single colour.

Question 9.
What is fundamental tone ?
Answer:
In a note, the sound of least frequency is known the fundamental tone.

Question 10.
What should be the frequency of audible sound ?
Answer:
We can hear sound only if the number of vibrations per second, called, frequency of a vibrating body lies within the limit 20 to 20,000.

Question 11.
Between a plane mirror and a wail of house, which one will reflect sound?
Answer:
A wall of house will reflect sound.

Question 12.
What is the minimum distance of the reflector to produce an echo ?
Answer:
The minimum distance of the reflector is 16.6 m to produce an echo.

Question 13.
What is Hertz ?
Answer:
The unit, used at present to express frequency is Hertz (Hz) named after the celebrated German physicist Heinrich Hertz.

Question 14.
What is decibel ?
Answer:
Decibel (dB) is the unit of expressing intensity of sound.

Question 15.
The oscillation of a pendulum cannot be heard. Why ?
Answer:
The oscillation of a pendulum has a frequency less than 20 Hz. So, we cannot hear the sound of Oscillation.

Question 16.
State the relation among velocity, wavelength and frequency of a wave.
Answer:
Wave velocity = Frequency x Wavelength

Question 17.
What is the unit of frequency ?
Answer:
Unit of frequency is cycles/second or CPS or Hz.

Question 18.
What is the unit of wavelength in SI system ?
Answer:
The unit of wavelength in SI system is metre.

Question 19.
What is the velocity of sound at 0°C ?
Answer:
332 m/sec.

Question 20.
Is medium required for the propagation of sound ?
Answer:
Medium is required for the propagation of sound.

Question 21.
Which is responsible for the generation of sound ?
Answer:
Vibration is responsible for the generation of sound.

Question 22.
How does pitch depend on frequency ?
Answer:
Greater the frequency, higher will be the pitch.

Question 23.
Can sound travel through vacuum ?
Answer:
Sound cannot travel through vacuum.

Question 24.
How many frequencies are there in a tone ?
Answer:
A tone is a sound of single frequency.

Question 25.
What is called fundamental tone ?
Answer:
In a note the tone of lowest frequency is called the fundamental tone.

Short Answers type Questions Marks for each 2

Question 1.
What is the definition of sound ?
Answer:
Sound: Sound is a type of energy, which is generated from a vibrating body, and which travels through an elastic medium and finally reaches our ear to create a special sensation in the brain.

Question 2.
How is the sound produced ?
Answer:
The material bodies capable of producing sound are called the sources of sound or sounding bodies. Sound is produced due to vibrations of the sounding bodies.

Question 3.
What is ultrasonic sound ?
Answer:
Ultrasonic sound : We cannot hear sound generated from the sources having frequency 20,000/sec or more. These sounds are known as ultrasonic sound.

Question 4.
What is subsonic sound ?
Answer:
Subsonic sound : We cannot hear the sound generated from the sources having frequency 20/sec or less. These sounds are known as subsonic sound.

Question 5.
What is audible sound ?
Answer:
Audible sound : We can hear only the sounds of the sources producing about 20 to 20,000 vibrations per second. These sounds are called audible sound.

Question 6.
Can sound travel through vacuum ?
Answer:
Explanation : For propagation of sound, a material medium (solid, liquid or gas) is necessary. Sound cannot travel through vacuum.

Question 7.
If a person talks or even screams on the surface of the moon, the sound will not be audible. Explain why.
Answer:
Explanation : The moon has no atmosphere or gas surrounding the moon. It is absolutely vacuum all around the moon. That is why if a person talks or even screams on the surface of the moon, the sound will not be audible.

Question 8.
Define wave.
Answer:
Definition of wave : It is defined as a disturbance that moves through a medium transferring energy from a point to another point without any physical transportation of the material between the points.

Question 9.
What is transverse wave ?
Answer:
Transverse wave: Transverse wave motion is that wave motion in which the individual particles of the medium execute vibrations about their mean positions in a direction perpendicular to the direction of motion of the wave.

Question 10.
What is longitudinal wave ?
Answer:
Longitudinal wave : Longitudinal wave motion is that wave motion in which the individual particles of the medium execute vibrations about their mean positions along the direction of motion of wave.

Question 11.
What do you mean by oscillation ?
Answer:
Oscillation: An oscillation of a particle is its trajectory in which the particle starting from a certain position moves through some distance and retracing the path it returns to its starting point with the same direction of motion with which it started initially.

Question 12.
What do you mean by amplitude ?
Answer:
Amplitude: The maximum displacement of a particle in a medium on either side of its mean position is called amplitude.

Question 13.
What is time period ?
Answer:
Time period : The time required by a particle to make one complete oscillation is known as time period. It is denoted by T.

Question 14.
What is frequency ?
Answer:
Frequency : The number of complete oscillations of a source of sound per second is known as its frequency.
Unit of frequency is Hertz, its symbol is Hz.

Question 15.
What do you mean by wavelength ?
Answer:
Wave length :

• It is the distance a wave or an energy travels through during one complete oscillation of its source.
• It is the distance between the two consecutive crests or troughs in a wave.
  It is normally represented by A. Its SI unit is metre (m).

Question 16.
What is wave velocity ?
Answer:
Wave velocity : In a medium, the distance travelled by the wave in one second is defined as the velocity of propagation of the wave or wave velocity. It is normally represented by c.

Question 17.
Establish a relation connecting wave velocity, wavelength and frequecy.
Answer:
Relation among wave velocity, wavelength and frequency :
Let the frequency and wavelength of a wave be v and λ respectively.
According to the definition :
‘v’ numbers of complete waves are generated in 1 sec. Since wavelength is λ, so in 1 sec, wave advances by v x λ = vλ distance.
Now, the distance traversed by wave in 1 sec is its velocity (v).
So,v = vλ
∴ Wave velocity = Frequency x Wavelength
This equation is applicable for all types of waves.

Question 18.
What do you mean by free or natural vibration ?
Answer:
Free or Natural vibration : When a body vibrates with its own natural frequency, it is said to execute natural or free vibrations. The frequency of free vibration depends on the density, shape and elasticity of the body.
Example : The frequency of a tuning fork depends on the length and cross-section of its arms as well as density and elasticity of the material of tuning fork.

Question 19.
What is called forced vibration ?
Answer:
Forced vibration: When a body is maintained in a state of vibration by a strong periodic force, the vibration is called forced vibration.

Example : Intensity of sound generated from a tuning fork is not very high and hence cannot be heard from a distance. Now when this tuning fork is touched on a table, it causes forced vibration on the table and thereby sound is generated from the table also. Intensity of this sound is relatively high and hence can be heard from a distant location.

Question 20.
What is called resonant vibration ?
Answer:
Resonant vibration: It is a particular case of forced vibration when the frequency of the driving force is equal to the natural frequency of the body. When such condition is fulfilled amplitude of oscillation becomes very high.
Example: A vibration is generated in the air column. When a vibrating tuning fork is placed on the mouth of a pipe filled with air, a shrill sound will be produced. Air being forced vibrated creates this sound.

Question 21.
What is meant by reflection of sound ?
Answer:
Reflection of sound : When sound waves hit on the boundary separating two homogeneous media, a portion of sound changes its direction from the surface of separation and returns to the first medium. This is known as reflection of sound.

Question 22.
State the laws of reflection of sound.
Answer:
Laws of reflection of sound :

• The incident sound, reflected sound and the normal on the surface of separation through the point of incidence remain on the same plane.
• The angle of incidence is equal to the angle of reflection.

Question 23.
What is mach number ?
Answer:
Mach number: The ratio of speed of a body to the speed of sound is called the mach number of the body. If the mach number of a body be greater than 1, then the velocity of the body becomes supersonic velocity.

Question 24.
How does speaking tube operate ?
Answer:
Speaking tube : If we talk at one end of a long tube, it can be heard clearly on the other end. The sound generated at one end does not get scattered. On the contrary, it undergoes several reflections within the tube and finally reaches the other end. In this way, a speaking tube operates.

Question 25.
How does a stethoscope function ?
Answer:
Stethoscope : Doctors use this gadget for chest examination. This is basically a reflecting tube. This gadget consists of two rubber tubes, with a metallic plate at the junction of the tube. The sound of the heart beat reaches from the metallic plate to the ears of the doctor through multiple reflections of the sound within the rubber tubes.

Question 26.
What is echo ? What is the condition for its occurrence ?
Answer:

• Echo : When a sound after reflection from some reflector is again heard separately from the original sound, then this reflected sound is known as an echo.
• Condition : For occurrence of an echo the minimum distance between the source of sound and a reflector of sound producing echo should be arround 16.6 m.

Question 27.
Deduce the minimum distance of a reflector that can produce echo of a transient sound.
Answer:
Minimum distance of the reflector for listening the echo of a transient sound : The sensation of a transient sound remains in our brain is \(\frac{1}{10}\) sec. after it reaches our ear. This period is known as persistence of hearing. If a second sound reaches our brain within this sec. of time, the later one cannot be identified by the brain separately. Obviously a gap of at least sec. \(\frac{1}{10}\) should be there between the original sound and reflected sound so that the brain can recognize them separately.

Now, if the velocity of sound in air be 332 m/sec, \(\frac{1}{10}\) in sec. it travels through 33.2 m. For producing an echo, the total length of path traversed by a sound from the source to the reflector and back should be at least 33.2 m. Hence for hearing echo of a transient sound, the minimum distance of the reflector from the source of sound is half of 33.2 m i.e. 16.6 m.

Question 28.
Give two applications of echo.
Answer:
Applications of echo :

• Determination of velocity of sound in air with the help of stopwatch and a pistol.
• Depth of sea, height of an aeroplane above ground may also be determined with the help of echo.

Question 29.
What is musical sound ?
Answer:
Musical sound : A continuous sound pleasing to the ear and produced by regular and periodic vibrations of a source is known as musical sound.
Example : Sounds generated from different musical instruments like violin, piano, flute etc. are musical sounds.

Question 30.
What is noise ?
Answer:
Noise : A noise is a sound which is discontinuous, unpleasant to the ear and produced by irregular and non-periodic vibrations of a source.
Example : The sounds of moving cars, trains, machines, sound of explosions etc. are the examples of noise.

Question 31.
What do you mean by tone ?
Answer:
Tone : A musical sound having a single frequency is known as tone.
Example : The sound generated from a tuning fork has a unique frequency. Hence the sound generated from a tuning fork is a tone.

Question 32.
What is meant by Note ?
Answer:
Note: A musical sound however generally consists of number of components having different frequencies. Such a sound is known as note.

Question 33.
What is fundamental tone and what is overtone ?
Answer:
Fundamental tone : In a note the tone of lowest frequency is known as fundamental tone.
Overtone : All tones, except fundamental tone, present in a note are known as overtones.

Question 34.
What is harmonics and what is an octave ?
Answer:

• Harmonics : Those overtones whose frequencies are integral multiples of the frequency of the fundamental tone are called harmonics.
• Octave : The particular harmonic whose frequency is double the fundamental frequency is known as the second harmonic or octave of the fundamental tone.

Question 35.
What are the characteristics of musical sound ?
Answer:
Characteristics of musical sound: Musical sound has three important characteristics. These are :

• Intensity
• Pitch and
• Quality or timbre

Question 36.
What is intensity of a musical sound ?
Answer:
Intensity : It means how forcefully or how strongly the sound reaches to a listener. Intensity or loudness of a sound depends on the energy contained per unit volume of the medium through which sound passes.

Question 37.
What are the factors which affect intensity of a musical sound ?
Answer:
The intensity of a musical sound depends upon the following factors :

• Amplitude of the vibration of the source
• Size of the sounding body
• Distance of the observer from the source
• Presence of other bodies
• Density of the medium.

Question 38.
What is pitch ? On what factory pitch of a note depends ?
Answer:

• Pitch: The characteristic of a musical sound by which a sharp or a shrill sound is distinguished from a dull, flat sound of the same intensity is known as its pitch.
• Pitch of a note depends: Pitch of a note depends on its frequency. Greater the frequency, higher will be the pitch. Wavelength being inversely proportional to frequency, the sound of low wavelength has higher pitch and vice versa.

Question 39.
What is quality of a musical sound ?
Answer:
Quality of a musical sound : It is the characteristic of a musical sound which enables us to distinguish one sound from another even though the two sounds may have the same intensity or pitch. Richer the quality, more pleasant is the sound.
Example : If harmonium and violin are played simultaneously, one can easily distinguish between these two even without seeing. This distinction is felt out of the property called quality of sound.

Question 40.
What is sound pollution ?
Answer:
Sound pollution : Any unpleasant and unwanted sound is noise. Sound pollution means creation of discomfort, disturbance and irritation which result to ill effects to mental and physical health.

Question 41.
What are the sources of sound pollution ?
Answer:
Sources of sound pollution :

• The sound of vehicle causes sound pollution.
• The machinery used in industries creates sound of intensity more than tolerance limit and thereby causes sound pollution.
• Loudspeakers, amplifiers, air horn played at top volume are major sources of sound pollution.
• The sound crackers cracked in different functions e.g. Puja ceremony, Marriage ceremony etc. causes sound pollution.

Question 42.
What are the harmful effects of sound pollution ?
Answer:
Harmful effects of noise pollution :

• Sound pollution may cause hinderance in the growth of nervous system of a child in the womb.
• One may become deaf due to sound pollution.
• The sound of high intensity and pitch may cause blood pressure, nervous breakdown, heart disease, loss of memory, loss of concentration etc.

Question 43.
What are the remedial measures of sound pollution ?
Answer:
Remedial measures of sound pollution :

• Factory should be prohibited in residential area.
• Sound resistant measures may be taken in industry.
• Unnecessary use of horn in the vicinity of hospital, school etc should be restricted.
• Airport should be away from residential area.
• Sound pollution awareness programme should be taken up at regular interval.

Question 44.
Why does the sound of explosion occuring in the sun not reach the earth ?
Answer:
Explanation: The continuous explosive sounds occuring in the sun because of conversion of hydrogen to helium by nuclear fusion is not heard from the earth. This is due to the presence of a large space beyond the earth’s atmosphere without any material medium. The sound of explosion cannot transmit to the earth since sound cannot propagate without the help of a material medium.

Question 45.
Why it is not advisable to regulate a watch on hearing the sound of a distant siren ?
Answer:
Explanation: The sound from the siren at a distance takes some time to reach a place. If the person adjusts his watch with the idea that the sound of the siren reaches him just at the time of its production, his watch will be slow by a few seconds that is taken by the sound to reach him.

Question 46.
Why are voices of females or babies more sharp than that of a grown up male person ?
Answer:
Explanation : Vocal cords of females or babies are tender, so, while they speak or sing the cords vibrate with high frequency, for this reason their voices are very sharp. Vocal cords of male persons are usually stiff, so their frequency of vibration is low and hence male voices are generally flat.

Broad answer type questions 

Question 1.
Why sound waves are called elastic waves ?
Answer:
Reason :

• Sound waves require some time to travel from one place to another. Just like on elastic wave.
• Vibration of sources are necessary for the production of elastic waves.
  Sound waves also require vibration of the sources for their production.
• The particles of the medium through which elastic waves pass, are not bodily moved when the waves pass through then. The same thing happens for the sound waves.
• Like all other elastic waves, sound waves require a material medium for their propagation.
• Sound waves like all other elastic waves undergo reflection, refraction and interference.

Question 2.
Describe in brief an experiment which shows that a material medium is necessary for propagation of sound.
Answer:
Experiment to prove
the necessity of material medium for propagation of sound :
Experimental setup: A glass jar is placed in airtight condition on a platform provided with a suction pump. An electric bell kept suspended inside the jar can be operated from outside with the help of a battery.

Experiment : The electric bell is set into operation. Air from inside the jar is gradually drawn out.

Observation : A sound of the bell is heard when the jar is full of air but the sound gradually fades away on withdrawal of air. The sound is again heard clearly when air is re-introduced.

Inference : Since sound from the electric bell is not heard in absence of air within the jar, it proves that sound from a source cannot propagate in absence of a material medium.

Numerical Problems :

Working formula]
(i) υ = vλ (υ = wave velocity,
v = frequency, λ = wavelength)

(ii) s = υ x t (s = Distance,
υ = velocity of sound
t = time)

(iii) υ_t = ν₀ (1 + 0.00 183t)
ν>₀ = velocity of sound at 0°C
υ_t= velocity of sound at t°C)

(iv) Height of an aeroplane above ground (h) = \(\frac{t}{2} \sqrt{v^2-u^2}\)
(υ = velocity of sound
u = velocity of aeroplane
t = time interval of sound and echo)

(v) \(v=\frac{s}{2}\left(\frac{1}{t_1}+\frac{1}{t_2}\right)\)
(v = velocity of sound
s = distance, t₁ initial time t₂ final time)

Problem 1:
The velocity of sound in air is 350 m/s. Frequency of a tuning fork is 700/s. Find the wavelength of the sound generated by the tuning fork.
Answer:
We know,
n = 700sec
λ = ?
y = 350 m/sec.
υ = vλ
Or, λ — \(\frac{v}{v}\)
= \(\frac{350}{700}\)=0.5m

Problem 2:
The frequency of a tuning fork is 440 Hz. If the wave length of the wave is 0.73 m then find the velocity of sound.
Answer:
v = 440 Hz
λ = 0.73m
υ =?
We know,
υ = vλ
or, v = 440 x 0.73
= 321.2 m/s

Problem 3:
A boat approaches a high vertical cliff. When the anchor is dropped, an echo is heard 2s later. How far is the boat from the cliff? (velocity of sound = 332 m/s)
Answer:
As the velocity of sound is 332 m/s so total distance travelled by sound in 2 s is = 2 x 332 = 664 m.
For an echo, the sound must go to the cliff and return through the same distance.
So the distance of the boat from the cliff = \(\frac{664 \mathrm{~m}}{2}=332 \mathrm{~m}\)

Problem 4 :
In measuring the depth of an ocean, it was found that the sound of an explosion and its echo by the bed had a time interval of 20s. between them
Answer:
As the velocity of sound in water = 1436 m/s, so total distance
travelled by the sound in 20s = 20 x 1436 m
The depth of the ocean = \(=\frac{20 \times 1436}{2} \mathrm{~m}\)
= 14360m

Problem 5 :
Find the frequency of a tuning fork whose wave length is 17m.? (velocity of sound in air = 340 m/s)
Answer:
υ= 340m/s
λ = 1.7m
v = ?
We know,
υ = vλ
or \(v=\frac{v}{\lambda}\)
= \(\frac{340}{1.7}\)
= 200 Hz
∴ Frequency of the tuning fork = 200 Hz

Problem 6 :
Two persons A and B are standing 1 km apart in open air. ‘A’ fires a gun and ‘B’ will receive the sound after 3 second of the flash. Calculate the velocity of sound in air.
Answer:
s = 1km = 1000m
t = 3
v = ?
We know,
\(v=\frac{s}{t}\)
or \(v=\frac{1000}{3}\)
∴ velocity of sound in open air = 333.3 m/s

Problem 7.
What should be the minimum distance of the reflector from the source of sound to hear the echo of the word ‘Beautiful’? (velocity of sound = 340 m/s)
Answer:
The word ‘beautiful’ is a trisyllabic one. So to hear it distinctly, one would require \(\left(3 \times \frac{1}{5}\right)\)  s = \(\frac{3}{5}\) = s to pronounce it.
In \(\frac{3}{5}\) sec sound travels \(\left(340 \times \frac{3}{5}\right)\) m = 204m
Thus, the minimum distance of the reflector from the source should be
= \(\frac{204 \mathrm{~m}}{2}\) = 102m

WBBSE Class 9 Physical Science Question Answer West Bengal Board

WBBSE Class 9 Physical Science Book Solutions West Bengal Board in English Medium

• Chapter 1 Measurement
• Chapter 2 Force and Motion
• Chapter 3 Matter: Structure and Properties
• Chapter 4 Matter: Atomic Structure, Physical and Chemical Properties of Matter
  • Chapter 4.1 Atomic Structure
  • Chapter 4.2 Mole Concept
  • Chapter 4.3 Solution
  • Chapter 4.4 Acids, Bases and Salts
  • Chapter 4.5 Separation of Mixtures
  • Chapter 4.6 Water
• Chapter 5 Work, Power and Energy
• Chapter 6 Heat
• Chapter 7 Sound

WBBSE Class 9 Physical Science Book Solutions West Bengal Board in Hindi Medium

• Chapter 1 मापन
• Chapter 2 बल और गति
• Chapter 3 पदार्थ : गठन तथा गुण
• Chapter 4 पदार्थ : संरचना, पदार्थ के भौतिक तथा रासायनिक गुण
• Chapter 5 कार्य, शक्ति एवं ऊर्जा
• Chapter 6 उष्मा
• Chapter 7 ध्वनि

WBBSE Class 9 Physical Science Syllabus West Bengal Board 2023

Chapter 1 Measurement
Measurement and Units: Concept of measurement of physical quantities, units and utility of units, Different Systems [CGS, SI] of Units, Units of different quantities in different systems, all physical quantities do not have units (reason), Fundamental Units and Derived Units, Modem ideas regarding standard length and standard time, Units of Mass (1 kg, 1 g), Unit of volume (1L, 1 m³, 1 mL, 1 cm³, 1 dm³).

Units of quantities having very large and very small magnitude: Sizes of living organisms, from microorganisms to large living species, size of the universe (with the help of the unit of length and unit of time) from subnuclear to galactic objects.

Dimension: Dimensions of physical quantities. Use of exponents in the expression of units.

Measurement: Measuring instruments – the idea of least count, range of the instrument, and error in case of the following instruments only – scale, clocks, measuring cylinder, common balance.

Chapter 2 Forces and Motion
Rest and Motion: Role of the observer in deciding rest and motion. Translation is a change of position and rotation is a change of orientation. Difference between rotation and circular motion.

Equations of Motion: Displacement, speed, velocity, and acceleration. Uniform and non-uniform motion among a straight line. Algebraic and graphical representation (through velocity-time graph only). Representation of displacement and acceleration in velocity-time graph.

• Derivation of the expression s = v.t. from the velocity-time graph.
• v = u + a.t from the velocity-time graph.
• Establishment of the expression s = \(\frac{1}{2}\).a.t² and s = u.t + \(\frac{1}{2}\).a.t² from the velocity-time graph, when the particle is moving with a uniform acceleration.
• Establishing the equation v² = u² + 2as from velocity-time graph.

Newton’s 1st Law of Motion: Force as the cause, and acceleration (change of motion) as an effect: (a) Effective force and balanced force. Addition of forces-Parallelogram law, examples from daily life. Resolution of forces, components examples, (b) Inertia of rest and inertia of motion.

Newton’s 2nd Law of Motion: Newton’s 2nd Law of Motion. Force is the cause, acceleration is an effect, and F = ma is the postulate (law). Mass is the intrinsic property of an object, determining internal resistance (using F = ma). Units of force (Dyne and Newton).

Newton’s 3rd Law of Motion: Newton’s 3rd Law of Motion. Action (force) and reaction (force).

Different types of forces: Forces of different types with examples. Tension is the force between different parts of a string or wire. The idea of normal reaction force.

Linear momentum: Introducing the concept of momentum for an object with variable mass. Necessity of linear momentum as a physical quantity. Statement of Newton’s 2nd law of motion by using the concept of linear momentum. Obtaining F = ma, from this statement when m is constant.

Conservation of linear momentum: Idea of conservation of linear momentum.

Chapter 3 Matter: Structure and Properties
The pressure of liquid and air: Fluid Pressure: Thrust, force per unit area, pressure of a liquid at a point, some characteristics of liquid pressure. Description of Barometer. Measurement of atmospheric pressure by Barometer and forecast of weather. Siphon and its applications; Archimedes’ Principle: Archimedes’ Principle. Buoyancy, floatation, and apparent weight. Density and Relative density; Surface tension: Idea of surface tension from examples of daily life. Factors affecting surface tension of a liquid; Viscosity: Viscosity is the friction of a liquid. Laminar or streamlined and turbulent flow. Density and viscosity are two different quantities. Terminal velocity of an object in a viscous medium. Rate of flow of liquid; Bernoulli’s Theorem: Bernoulli’s Theorem; Elasticity: Elasticity, factors on which elasticity depends, Stress, Strain, Hooke’s law, Elastic constants (Young’s modulus only), and Elasticity in daily life. Ductility; Brittleness; Malleability.

Chapter 4 Matter: Atomic Structure; Physical and Chemical Properties of Matter

Chapter 4.1 Atomic Structure:
Discovery of the electron; Rutherford’s alpha particle experiment; Rutherford’s model of an atom; Limitations of Rutherford’s model; Discovery of Neutron; Electron, proton, and neutron; Qualitative presentation of Bohr-Rutherford model of an atom. Electronic orbits (K, L, M, N, shells); Isotope, isobar, and isotone; Nuclear force; Electronic configuration upto Z = 20; Absorption/emission of energy during electronic transitions between orbits.

Chapter 4.2 Mole Concept:
Mole as a unit of number. Avogadro’s number (NA) and its importance in chemistry, biology, and physics; Concept of gram atom and gram molecule; Atomic mass unit; Molar volume of gases at NTP; Use of molar mass, molar volume, and formula mass in chemical calculations.

Chapter 4.3 Solution:
True solution, colloidal solution, and coarse suspension; Diameter of particles in true and colloidal solutions and coarse suspension; Dissolution of small ions/molecules and macromolecules (protein, DNA, starch) in water; Different colloidal solutions (solid-in-liquid, solid-in-gas, liquid-in-gas, liquid-in-liquid). Emulsions and emulsifiers; Solubility of solids and gases in liquids; Saturated, unsaturated, supersaturated solutions; Crystallization; Concentration of solution and its units; Motion of particles in solution; Non-aqueous solvent.

Chapter 4.4 Acids, Bases, and Salts:
Arrhenius definition of acids and bases; Indicators; Industrial uses of NaOH, H₂SO₄, HCl, HNO₃; Chemical properties of acids (H₂SO₄, HCl, HNO₃; Chemical properties of alkali (NaOH); Safe handling of acids and bases; Importance of aqueous medium in respect of acid-base property; Qualitative introduction to pH; Effect of pH on tooth decay; importance of pH in agriculture and pisiculture; Acidic, basic and amphoteric oxides; Acidification of water due to dissolution of gaseous oxides (CO₂, SO₂, NO₂), Neutralization, Antacids, Normal, acidic and basic salts.

Chapter 4.5 Separation of Components of Mixtures:
Distillation and Fractional distillation; Use of Separatory funnel.

Chapter 4.6 Water:
Physical properties of water that influenced proliferation and maintenance of life; Quality parameters of drinking water; Purification of water for drinking purposes; Soft and hard water; Water pollution; Arsenic compounds and fluoride in groundwater.

Chapter 5. Energy in Action. Work Power and Energy
Work: Definition of work, W = F.D. Unit of work. Work done by and against the force. No workforce; Power: Definition of power, p = w/t. Unit of power; Energy: Definition of energy. Potential energy. Kinetic energy. Conservation of Mechanical Energy.

Chapter 6 Heat
Calorimetry: Principles of calorimetry. Heat gained = Heat lost; Equivalence of work and heat: W = J.H; Latent heat: Change of state; Saturated and unsaturated vapour: Saturated and unsaturated vapour; Anomalous expansion of water: Effects of anomalous nature of water on marine life.

Chapter 7 Sound
Sources of sound: Vibration: Sound is produced by vibration. Mechanism of sound production in the human vocal chord. Some quantities are related to vibration amplitude, period, and frequency; Propagation of sound Wave: Necessity of medium for sound propagation; Waves: Longitudinal and Transverse wave. Some physical quantities related to waves: are amplitude, wavelength, period, wave frequency, and wave velocity. Relation among wave frequency, wavelength, and wave velocity V = n.λ; Some properties of sound: Reflection of sound. Echo. Reverberation of sound; Audible sound and its frequency range. Ultrasonic sound. Uses of ultrasonic sound; Characteristics of Sound: Loudness. Pitch. Quality of Sound.
The human ear and mechanism of hearing sound: Structure (Human ear). Sound Propagation through the different parts of the ear; Sound Pollution: Sound Pollution and its remedy.

WBBSE Class 9 Physical Science Blueprint for 1st Summative Evaluation (Total Marks – 40)

Theme/Sub-Theme	MCQ (Group A)	VSA (Group B)	SA (Group C)	LA (Group D)	Total
1. Measurement	1 × 3	1 × 2	2 × 2	3 × 1	12
2. Force and Motion	1 × 2	1 × 3	2 × 3	3 × 1	14
3. Atomic Structure	1 × 3	1 × 2	2 × 3	3 × 1	14
Total	8	7	16	9	40

WBBSE Class 9 Physical Science Blueprint for 2nd Summative Evaluation (Total Marks – 40)

Theme/Sub-Theme	MCQ (Group A)	VSA (Group B)	SA (Group C)	LA (Group D)	Total
1. Mole Concept	1 × 2	1 × 1	2 × 1	3 × 1	8
2. Matter: Structure and Properties	1 × 2	1 × 3	2 × 1	3 × 1	10
3. Solution	1 × 1	1 × 1	2 × 1	3 × 1	7
4. Acids, Bases, Salts	1 × 2	1 × 1	2 × 1	3 × 1	8
5. Work, Power, Energy	1 × 1	1 × 1	2 × 1	3 × 1	7
Total	8	7	10	15	40

1. In Group A: All the Multiple Choice Questions (MCQ) are compulsory. There will be no alternative to any question in this Group.
2. In Group B: VSA will contain – (i) answer in a single word or a single sentence, (ii) column matching, (iii) fill in the blanks, and (iv) true/ false type questions. In the first & second summative, there will be alternatives to a total of 3 questions from the same theme/sub-theme.
3. In Group C: In the first & second summative there will be an alternative to 3 questions from the same theme/sub-theme.
4. In Group D: In the first & second summative there will be an alternative to 3 questions from the same theme/sub-theme. In this Group, 3 marks may be broken as (2 + 1).
5. All alternatives should be internal i.e. an alternative to question (2a) should be designed as (2a) [Question] OR [Question], (2b), etc.
6. Each numerical question will have an alternative item from the same theme/sub-theme.

WBBSE Class 9 Physical Science Blueprint for 3rd Summative Evaluation/Selection Test (Total Marks – 90)

Section	Theme/Sub-Theme	MCQ (Group A)	VSA (Group B)	SA (Group C)	LA (Group D)	Total
Physics	1. Matter: Structure and Properties	1 × 1	1 × 1	2 × 2	3 × 1	9
	2. Measurement	1 × 1	1 × 2	2 × 1	–	5
	3. Force and Motion	1 × 1	1 × 2	2 × 1	3 × 1	8
	4. Work, Power, Energy	1 × 1	1 × 2	2 × 2	–	7
	5. Sound	1 × 1	1 × 2	2 × 1	3 × 1	8
	6. Heat	1 × 1	1 × 2	2 × 1	3 × 1	8
Chemistry	7. Atomic Structure	1 × 1	1 × 2	2 × 1	3 × 1	8
	8. Mole Concept	1 × 1	1 × 2	2 × 1	3 × 1	8
	9. Solution	1 × 1	1 × 2	2 × 1	3 × 1	8
	10. Acids, Bases and Salts	1 × 2	1 × 2	2 × 1	3 × 1	9
	11. Separation of Components of Mixtures	1 × 1	1 × 2	2 × 1	–	5
	12. Water	1 × 1	1 × 2	2 × 2	–	7
Total		13	23	30	24	90

 

1. In Group A: All the Multiple Choice Questions (MCQ) are compulsory. There will be no alternative to any question in this Group.
2. In Group B: VSA will contain – (i) answer in a single word or a single sentence, (ii) column matching, (iii) fill in the blanks, (iv) true/ false type questions. In this group there will be alternatives to a total of 8 questions: alternative to 4 questions from Physics and 4 questions from Chemistry will be given. All alternatives will be from the same theme/sub-theme.
3. In Group C: (a) Eight (8) questions from Physics will have to be answered. There will be alternatives to 3 questions from the same theme, (b) Seven (7) questions from Chemistry will have to be answered. There will be an alternative to 3 questions from the same sub-theme.
4. In Group D: (a) Four (4) questions from Physics will have to be answered. There will be alternatives to 2 questions from the same theme, (b) Four (4) questions from Chemistry will have to be answered. There will be an alternative to 2 questions from the same sub-theme. In this Group, 3 marks may be broken as (2 + 1).
5. All alternatives should be internal i.e. an alternative to question (2a) should be designed as (2a) [Question] OR [Question], (2b), etc.
6. Each numerical question will have an alternative item from the same theme/sub-theme.

WBBSE Class 9 Solutions

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 1 Measurement offer valuable context and analysis.

WBBSE Class 9 Physical Science Chapter 1 Question Answer – Measurement

Very short answer type questions

Question 1.
What is a physical quantity ?
Answer:
A physical quantity is any measurable quantity of an object or an event.

Question 2.
For clear understanding what must be present in the result obtained after measuring a physical quantity ?
Answer:
Unit.

Question 3.
By which instrument we to measure the weight of a body ?
Answer:
Spring balance.

Question 4.
Is litre a fundamental unit or a derived unit ?
Answer:
Derived unit.

Question 5.
What is the unit of atomic weight ?
Answer:
No unit.

Question 6.
Is light-year a fundamental or a derived unit ?
Answer:
Fundamental unit.

Question 7.
How many fundamental physical quantities are present in the derived physical quantity momentum ?
Answer:
Three. They are Mass, Length, Time.

Question 8.
Is work done by a force a scalar or a vector quantity ?
Answer:
Scalar quantity.

Question 9.
Which unit should be chosen to measure the distance between two stars ?
Answer:
Light year.

Question 10.
What is the SI unit of temperature ?
Answer:
Kelvin.

Question 11.
Write down a physical quantity which has no unit ?
Answer:
Atomic weight.

Question 12.
What type of time measuring device is used in a race?
Answer:
Stop-watch.

Question 13.
Which physical quantity is measured by a common balance?
Answer:
Mass.

Question 14.
What is the type of unit of velocity ?
Answer:
Derived unit.

Question 15.
Whose unit is millilitre ?
Answer:
Volume.

Question 16.
What is the unit of length in SI system ?
Answer:
Metre.

Question 17.
Name one physical quantity whose unit is formed by using all the three fundamental units.
Answer:
Force.

Question 18.
Which instrument is usually used to measure length ?
Answer:
Ordinary scale.

Question 19.
What is the unit of density in SI system ?
Answer:
kg m³.

Question 20.
What is the unit of volume in CGS system ?
Answer:
cm³

Question 21.
Is unit of area a fundamental unit ?
Answer:
No.

Question 22.
How many seconds are there in a mean solar day ?
Answer:
86,400 seconds.

Question 23.
What must be mentioned in the result obtained after measuring a physical quantity apart from a number?
Answer:
Unit must be mentioned in the result.

Question 24.
Name two physical quantities having same unit ?
Answer:
Speed and velocity.

Question 25.
Why is the temperature 4°C mentioned in defining density of water inCGS system ?
Answer:
Because at 4°C, water has maximum density which is 1 c.c. and water has the mass 1g.

Question 26.
Arrange in ascending order of magnitude : 1 micron, 1 angstrom, 1 fermi.
Answer:
1 fermi < 1 angstrom < 1 micron.

Question 27.
What is the unit of refractive index?
Answer:
No unit.

Question 28.
Write down the SI unit of momentum.
Answer:
kg-m/sec

Question 29.
How many metre is one Fermi ?
Answer:
1 Fermi =10^-15 m.

Question 30.
Give an example of a scalar quantity which is the product of two vector quantities.
Answer:
Work.

Question 31.
What is the dimention of force?
Answer:
[MLT^-2].

Question 32.
Name a physical quantity which has a unit but no dimension.
Answer:
Angle.

Question 33.
Which unit should be chosen to measure distance between two planets ?
Answer:
Light year.

Question 34.
Write down the dimension of density.
Answer:
[ML^-3].

Question 35.
Is the unit of volume a fundamental or a derived unit ?
Answer:
Derived unit.

Question 36.
Write down theCGS unit of pressure.
Answer:
dyne/cm².

Question 37.
What is the SI unit of mass ?
Answer:
Kilogram.

Question 38.
Give the name of two scalar quantities which have same unit.
Answer:
Work and energy. Both have SI unit Joule andCGS unit erg.

Question 39.
What is the mass of 1 ml water at 4°C ?
Answer:
1 g.

Question 40.
The density of Hg is 13.6 g cm³. What is the density of Hg in SI system ?
Answer:
13600 kg m³.

Question 41.
At which temperature water has maximum density ?
Answer:
4°C.

Question 42.
Where is chronometer used ?
Answer:
The mariners use the chronometer for finding out longitude of a plane.

Question 43.
What is the dimension of velocity ?
Answer:
[LT^-1]

Question 44.
What is the dimension of retardation ?
Answer:
[LT^-2]

Question 45.
Which physical quantity has the dimension [MLT^-2] ?
Answer:
Force.

Question 46.
What is the SI unit of Force ?
Answer:
Newton.

Question 47.
Which physical quantity is meåsured by spring balance?
Answer:
Weight.

Question 48.
Give an example of a vector quantity which is derived from only one fundamental unit.
Answer:
Displacement.

Question 49.
The word ‘Science’ has its origin in a Latin word. Name it.
Answer:
Scientia.

Question 50.
The word ‘physics’ comes from a Greek word. What is it ?
Answer:
Physis.

Question 51.
What is measured by astronomical units ?
Answer:
Is is used in measuring astronomical distances.

Question 52.
Is ‘par sec’ a unit of time?
Answer:
No. It is the distance at which an arc of length of 1 AU (Astronomical unit) subtends an angle of one second of an arc.

Question 53.
What is the dimension of area?
Answer:
Dimension : The dimensions of a physical quantity are expressed as the powers to which the fundamental units of mass, length and time are raised to obtain the derived unit of the quantity.

Question 54.
What do we measure with a measuring cylinder?
Answer:
A measuring cylinder is used for measuring the volume of a liquid.

Question 55.
Name the device used in measuring weight of a body.
Answer:
Spring balance.

Question 56.
What does a stop watch measure ?
Answer:
Small time interval is measured by stop watch.

Question 57.
What is the multiplicative factor micro (mu) ?
Answer:
10^-6.

Question 58.
What do we measure with a common balance ?
Answer:
We measure the mass of a body usually with a common balance.

Short Answers type Questions

Question 1.
What is the necessity of measurement of a physical quantity ?
Answer:
Necessity of measurement of a physical quantity.
Measurement of a physical quantity gives a clear idea about largeness or smallness of the quantity. Simple eye estimation does not give any exact size of the physical quantity. For this reason, measurement of a physical quantity is very important.

Question 2.
What is physical quantity?
Answer:
Physical quantity : Any measurable physical entity is known as physical quantity.
e.g. length, mass, time etc.

Question 3.
What is scalar quantity ?
Answer:
Scalar quantity : The physical quantity which has only magnitude but no direction is called scalar quantity.
e.g. length, area, volume, mass etc.

Question 4.
What do you mean by vector quantity?
Answer:
Vector quantity : The physical quantity which has both magnitude and direction is called vector quantity.
e.g. displacement, velocity, force, weight etc.

Question 5.
What is unit ?
Answer:
Unit : Unit is the universally accepted definite amount of a physical quantity taken as a standard for the measurement of the same physical quantity of any amount.

Question 6.
What are fundamental units ?
Answer:
Fundamental units : The units of physical quantities like length, mass and time which do not depend on each other and from which units of other physical quantities may be derived, are called fundamental units.

Question 7.
What are derived units ?
Answer:
Derived units : Those units which are derivable from one or more of the fundamental units are known as derived units.
e.g. units of acceleration, force, speed, momentum etc. are derived units.

Question 8.
Has every physical quantity a unit ?
Answer:
No, there are some physical quantities which are measured by the ratio of two identical quantities.
e.g. atomic weight, specific heat, specific gravity etc.
These physical quantities have no units, they are purely numerical numbers.

Question 9.
What are the characteristics of fundamental units ?
Answer:
Characteristics of fundamental units :

1. They do not depend on each other.
2. They can form the units of other physical quantities.
3. They are only three in numbers.

Question 10.
What are the types of different system of units ?
Answer:
Different system of units : At present two different system of units are used in science throughout the world.
(a) CGS or Metric system.
(b) SI or international systems of units (1960).

Question 11.
What isCGS system ?
Answer:
CGS or French or Metric system : This system was originated in France but now it is used in the scientific measurements and is widely used throughout the world including India. From the initial letters of the words centimetre, gram and second, this system is calledCGS system. This is a decimal system.

Question 12.
What is SI unit ?
Answer:
SI or International System of units : In 1960, an international system of units of measurement known as SI system was recommended by the GeneralConference on Weights and Measures in order to have a consistent system of units and to simplify communications among the scientists.

Question 13.
Define metre.
Answer:
Metre : The distance between the two marks made on a platinum-iridium (90: 10) bar maintained at 0°C temperature preserved at International Bureau of Weights and Measures at Sevres near Paris considered as one metre (symbol : m).

Question 14.
What is the modern concept of metre ?
Answer:
Modern concept of metre (October, 1983) :
The distance travelled by light in \(\frac{1}{3 \times 10^8}\) sec. in vacuum is called 1 metre.

Question 15.
State different smaller units of length in SI system.
Answer:
Smaller units of length in SI system :

1. Femtometre or Fermi (=10^-15 m)(fm)
2. Picometre (=10^-12 m)(pm)
3. X-ray unit (=10^-13 m)(Xu)
4. Angstrom (=10^-10 m)(A°A)
5. Nanometre (=10^-10 m)(nm)
6. Micrometre or micron (=10^-6 m)
7. Centimetre (=10^-2 m)(cm)
8. Decimetre (=10^-1 m)(dm)

Question 16.
What are the bigger units of length ?
Answer:
Bigger units of length :
(i) Astronomical unit (AU): The average distance between the Earth and the Sun.
1 AU = 14.95 × 10⁸ km
(ii) Light year : The distance that light wave travels in year in vacuum.
1 light year = 9.46 × 10¹² km
(iii) Persec : The biggest unit for the measurement of distance. 1 persec = 3.26 light year = 30.84 × 10¹² km.

Question 17.
What do you mean by mass ?
Answer:
Mass : Mass of a body is the quantity of matter contained in it.

Question 18.
State the bigger units of mass.
Answer:
Bigger units of mass :

1. Quintal (= 100 kg)
2. Metric ton or tonne (= 1000 kg = 10 Quintal)
3. CSL (= 1.4 × mass of the Sun)

Question 19.
State the smaller units of mass.
Answer:
Smaller units of mass :

(i) amu(= 1.6603 × 10^-24g)
(ii) 1 carat = 200 mg

Question 20.
What is density ? State itsCGS and SI units.
Answer:
Density : Density of the material of a body is the quantity of its mass per unit volume.
Unit of density inCGS system : g cm^-3
Unit of density in SI system : kg m^-3

Question 21.
What is Solar day?
Answer:
Solar day : The interval of time between two consecutive apparent transits of the Sun across the meridian at any place is called a Solar day.

Question 22.
What do you mean by mean Solar day ?
Answer:
Mean Solar day : The length of the Solar day varies from day to day owing to various reasons and the mean value of the actual Solar days averaged over a full year is called the mean Solar day.

Question 23.
What is the definition of second in SI system ?
Answer:
Definition of second in SI system : The time taken for the number of vibrations 9,192,631,770 of a radioactive atom Cs¹³³ in standard magnetic field is called one second.

Question 24.
What do you mean by volume?
Answer:
Volume : It is defined to be the space occupied by a substance. Solids have three dimensions i.e. length, breadth and height.

Question 25.
What is litre?
Answer:
Litre : Volume of 1 kg of pure water at 4°C or 277 K is called a litre.

Question 26.
What are the advantages of metric system ?
Answer:
Advantages of metric system :

1. In metric system, multiples and sub-multiples of the units of length and mass are always ten times the preceding ones.
2. In metric system, units of length, mass and volume are related by same relation.
3. In metric system, the same prefixes (such as milli, centi, deci etc.) are used in the units of length and mass.

Question 27.
What is the definition of dimension ?
Answer:
Definition of dimension : Dimensions of a derived unit are the powers to which the fundamental units of mass, length and time must be raised to present that unit.

Question 28.
What is the definition of dimensional expression ?
Answer:
Definition of dimensional expression : Dimensional expression is a product or quotient of the symbols of fundamental physical quantities, involved in a derived physical quantity raised to appropriate powers.
e.g. Dimensional expression of force : [MLT^-2].

Question 29.
State physical quantities without dimension.
Answer:
Physical quantities without dimension : A physical quantity is expressed as a ratio between two physical quantities of same unit becomes a number only, which will not have dimension.
e.g. Mechanical advantage of a machine, specific gravity, refractive index etc. are dimensionless. Again, angle although has unit (radian), it has no dimension.

Question 30.
What is the function of ordinary scale ?
Answer:
Function of ordinary scale : We generally use ordinary scale for the measurement of length.

Question 31.
How can you measure the length of a curved line ?
Answer:
Measurement of the length of a curved line: If the length of a curved line is to be measured with the help of a scale, we make use of a thread and place it along the curved line and measure the length of the straightened thread with the help of a scale.

Question 32.
Why scales are made usually with wood or plastic ?
Answer:
Scales are made
usually with wood or plastic: In making an ordinary scale wood or plastic is usually used instead of metal with the change in atmospheric temperature metal scales change in length. A metal scale gives correct readings only at a temperature at which it has been graduated. At any other temperatures it would give incorrect measurements. But in the case of wood or plastic these changes are negligible. So the scales are made usually with wood or plastics.

Question 33.
What is common balance?
Answer:
Common balance : In the laboratory we measure the mass of a body with a common balance. The weighing balance used in our daily life is simple version of this common balance.

Question 34.
What is the working principle of measurement of mass ?
Answer:
Working principle of measurement of mass : Actually we find the mass of the body by comparing its mass with some standard weights. Generally the measurable object as kept on the left pan of the balance and the standard weights are placed on the right pan. When the balance beam is horizontal, then
Mass of the object = Mass of the Standard Weights

Question 35.
What are the requisites of a good balance?
Answer:
Requisites of a good balance :

1. The balance must be true, i.e. the balance beam should be horizontal with no loads or with equal loads on the pans.
2. The balance must be sensitive.
3. The balance must be stable i.e. the balance beam must came quickly to its equilibrium position once it is displaced.
4. The balance must be rigid i.e. all the parts of the balance must. be strongly built.

Question 36.
What is a sensitive balance ? What are the conditions to be a sensitive balance?
Answer:
Sensitive balance : A balance is said to be sensitive when even for slight difference in weights on the two pans, the beam should be tilted from its horizontal position.
Conditions for balance to be sensitive :
(i) The balance beam should be light with long arms.
(ii) The centre of gravity of the beam should be near the fulcrum.

Question 37.
What is spring balance?
Answer:
Spring balance: With the help of this instrument weight of a body can be measured.

Question 38.
What is the working principle of measurement of weight ?
Answer:
Working principle of measurement of weight : The spring balance should be kept vertical and the pointer should be coincide with the ‘0’ mark of the scale. When a weight is suspended from the hook, the spring elongates, elongation being proportional to the weight. So, the pointer attached to the spring moves over the graduated scale. By noting its position against the scale, we can directly measure the weight of a body.

Question 39.
What is measuring cylinder ?
Answer:
Measuring cylinder: The instrument by which the volume of a liquid is measured is known as measuring cylinder.

Question 40.
What points should be noted during measuring the volume of liquid?
Answer:
Points should be noted :
(i) The volume of a liquid cannot be measured by measuring cylinder which reacts with glass.
(ii) The volume of a liquid cannot be measured by cylinder which is very much volatile.

Question 41.
How can you measure the density of an irregularly shaped body by using common balance and measuring cylinder ?
Answer:
Density : Density of a substance is defined as mass per unit volume of the substance.
If M be the mass of the substance, V the volume, then the density D is given by D = MV
Mass of the irregularly shaped body is determined by a common balance and volume is measured by a measuring cylinder. From equation (1) density of the body can be determined.

Question 42.
How will you determine the volume of a solid by a measuring cylinder?
Answer:
Determination of the volume of a solid by a measuring cylinder: To find the volume of the solid, some liquid in which the solid sinks but does not dissolve is taken in a dry measuring cylinder. The reading of the upper meniscus of the liquid in the cylinder is noted, let it be V₁ ml . The solid substance tied with a waxed thread is now slowly immersed in the liquid. Let the reading of the upper meniscus of the liquid in a measuring cylinder be V₂ ml. So, the volume of the given solid is (V₂-V₁) ml.

Question 43.
What precautions should be taken while determining volume and density of a solid body with the help of a common balance?
Answer:
Precautions:

1. The body must sink completely in the liquid taken in the measuring cylinder.
2. The body must not react chemically with the liquid and also it must not dissolve in the liquid.
3. While dipping the body in the liquid taken in measuring cylinder, it should be done carefully so that no liquid splashes out.

Question 44.
Is density of a material dependent on its mass or volume?
Answer:
Density of a substance is independent of its mass or volume. If the mass of a substance is increased or decreased, its volume correspondingly increases or decreases proportionately and vice versa. So, density, the ratio of mass to volume remains unaltered.

Question 45.
Why is temperature kept at 4°C is measuring the volume of a liquid in litre unit ?
Answer:
Density of pure water is maximum at 4°C and it varies with the change of temperature and so also the volume. Hence, temperature of 4°C is specified in litre unit.

Question 46.
Why one does not express his age in second?
Answer:
A person does not express his age in seconds, because with such a small unit of time the number expressing his age will be very large that gives no clear idea about the age. Above all, in calculating age in seconds and then expressing it with a large number, some more seconds will pass off which do not come into the calculation. So, the correct age of one cannot be expressed in seconds.

 Broad answer type Questions

Question 1.
Distinguish between scalar and vector quantities.
Answer:
Difference between scalar and vector quantities :

Scalar guantity	Vector quantity
(i) Scalar quantities have only magnitude and no direction.	(i) Vector quantities have both magnitude and direction.
(ii) Scalar quantities change due to change of magnitude only.	(ii) Vector quantities change due to change of magnitude or direction or both.
(iii) So far as addition, subtraction, multiplication etc. are concerned, scalar quantities obey algebraic laws.	(iii) So far as addition, subtraction multiplication etc. are concerned, vector quantities do not obey algebraic laws.

Question 2.
Write down the names and symbols of the fundamental units in SI system.
Answer:
Various fundamental physical quantities in SI system :

Question 3.
What are the advantages of metric system ?
Answer:
Advantages of metric system :
(i) In metric system, multiples and sub-multiples of the units of length and mass are always ten times the preceding ones. Thus the system is known as decimal system.In this system conversion of different multiples and sub-multiples of the units could be done by shifting the decimal point to the right or left.
(ii) In metric system, units of length mass and volume are related by simple relation.
(iii) In metric system, the same prefixes (such as milli, centi, deci etc.) are used in the units of length and mass

Question 4.
Why is temperature mentioned in the unit of length but not in the unit of mass ?
Answer:
The length is measured by a scale made of metals, plastic, wood or Gutta-percha etc. The length of these substances vary with difference of temperature. So in determining the unit of length temperature is to be specified to get the exact length. On the other hand mass being on intrinsic property of matter it does not vary with the change of temperature. So it is not necessary to specify the temperature.

Question 5.
How can you measure the exact mass of a solid by a common batance with unequal arm length ?
Answer:
Suppose the length of left and right arm of the balance are l₁ and l₂ respectively.
Now, keeping the solid with a mass of in at the right hand pan it is counterpoised with a mass w₁ at the left hand pan.
Now, w₁ × l₁ = m × l₂
or, \(\frac{l_1}{l_2}=\frac{m}{w_1}\) ….. …. (i)

Again, the solid of mass ‘m’ is kept at the left hand pan and supposed it is counterpoised with a load of mass w₂ at the right hand pan.

Question 6.
How can you measure the diameter of a piece of hair or thread?
Answer:
Measurement the diameter of a piece of hair or thread : To find diameter of a piece of hair or thread, a long piece of hair or thread is taken. The piece is then coiled over the millimetre marks on the scale. The coiling should be such that each turn remains in close contact of its adjacent one. Thus the added diametre of the segments of hair or thread in all the turns cover a few millimetre of the scale which is noted. Now dividing the length covered by the turns of hair or thread on the scale by the number of turns, diameter of the hair or thread is determined.

Question 7.
In theasuring certain length with a ruier why different portions of the ruier are made co-incident with the two ends of the given length ?
Answer:
As the graduation marks on the scale may not be uniformly spaced, due to manufacturing defects, several portions of the scale should be used i.e. the left end of the line may be made co-incident with different arbitrarily chosen marks of the scale like, 43 cm mark, 5.1 cm mark, 7.2 cm mark etc. The average of the several readings obtained after the above mentioned operations gives the required length of the line almost correctly.

Question 8.
How can you find the radius of a drop of a liquid ?
Answer:
Decermination of the radius of a drop of a hiquid: In order to find the radius of a drop of a liquid, a large number, say 200 drops of the liquid are taken in a dry measuring cylinder of small diameter. The drops of the liquid are taken with the help of a burette. The volume of the liquid is observed from the measuring cylinder. Now, it is divided with the number of drops of liquid taken when the volume of one drop is determined, then v ml = \(\frac{4}{3}\) πr₃, where π = 3.14.
So, the radius of the drop of liquid is determined.

Numerical Problems

Working formula :

(i) Volume of a rectangle solid = length × breadth × height
= l × b × h
(ii) Volume of a cube =(length)₃=l₃
(iii) Volume of a sphere = \(\frac{4}{3}\) π(radius )₃ = \(\frac{4}{3}\) πr₃
(iv) Volume of a cone = \(\frac{1}{3}\) π × (radius of base)² × perpendicular height
= \(\frac{1}{3}\) πr² h(π = \(\frac{22}{7}\))
(v) Volume of a right circular cylinder =πr² h
(vi) Density of a substance (D) = \(\frac{\text { Mass of the substance }(M)}{\text { Volume of the substance }(V)}\)

Example 1 : The mass of a proton is 1.67 × 10-27 kg. How many protons would be required to make 1 kg ?
Answer:

Example 2: The radius of an atom is abon: 1 A while that of the nucleus is about 1 fermi. How does the volume of the atom compared with that of the nucleus of the atom:
Answer:
1A° = 10^-10 m, 1 fermi = 10^-15 m
So, from the problem we get,

Therefore, volume of the atom is 10¹⁵ times greater than that of the nucleus.
Example 3 : Light of a certain colour is composed of a train of waves, each of 0.3048 micron long. How mans of the are are there in a metre?
Answer:
1 micron =10^-6 m
∴ Number of waves

Example 4 : Mankind has existed for about 106 years, whereas the universe is about 1010 years old. If the age of the universe is taken to be one day, for how many seconds has mankind existed?
Answer:
From the problem we get,

10¹⁰ years =24 × 60 × 60 sec.

Example 5 : Mass of a piece of gold in air is 193 g. In a measuring cylinder there is 50 cm 3 of water. When the piece of gold is immersed in water in the measuring cylinder, the reading of the volume of water is found to be 60 cm 3. What is the density of gold?
Answer:
From the problem we get, Volume of the piece of gold =(60-50) cm³=10 cm³

Example 6: If the mass and volume of a piece of iron be 760 g and 100 cm³, then what would be its density?
Answer:
Density of iron = \(\frac{\text { mass of iron }}{\text { volume of iron }}\)
= \(\frac{760 \mathrm{~g}}{100 \mathrm{~cm}^3}\) = 7.6 g/cm³

Example 7 : The volume of a can is 100 ml and it weighs 20 g. It is then filled fully by a liquid and weighs 180 g. Find the density of the liquid.
Answer: Volume of the liquid (v) = 100 ml
Mass of the liquid (M) = (180-20) g = 160 g

Example 8: The density of a liquid is 1.2 g / cm³. What will be the mass of 100 cm³ of the liquid?
Answer: Volume of the liquid =100 cm³
Density of the liquid =1.2 g / cm³
We know,

Mass of liquid =volume of the liquid × density of the liquid
= (100 × 1.2)g = 120g

Example 9 : The density of silver in CGS unit is 10.5 g / cm³. What will be the density of silver in SI unit?
Answer: From the problem we get, density of silver in CGS unit =10.5 g / cm³
So, the density of silver in SI unit
= 10.5 × 1000 kg / m³
= 10500 kg / m³

Example 10: The radius of a solid sphere is 10 cm. The density of its matter is 8 g / cm 3. What is the mass of the sphere?
Answer:
We know,
volume of the sphere V = \(\frac{4}{3}\) πr³(r= radius of the sphere)
∴ \(\frac{4}{3}\) × \(\frac{22}{7}\) × (10)³ cm³ ; r = 10cm
Density of its matter = 8 g/cm³
So, mass of sphere = volume × density
= \(\frac{4}{3}\) × \(\frac{22}{7}\) ×(10)³ × 8g=33.51 kg

Example 11.
Mass of a piece of solid substance is 20.52 g. When the piece is immersed in water its mass becomes 12.48 g. What will be its volume and density?
Answer:
Mass of displaced water by the body =(20.52-12.48) g=8.04 g
Volume of displaced water = \(\frac{8 \cdot 04}{1 \mathrm{~g} / \mathrm{cc}}\) = 8.04 c.c.
So, the volume of the solid substance = 8.04 c.c.
Density = \(\frac{20 \cdot 52}{8 \cdot 04}\) g / c.c. =2.55 g / c.c.

Example 12.
Find the dimension of force.
Answer:

Example 13.
Specific gravity is the ratio of two densities. Show that it has no dimension.
Answer:
Specific gravity of a substance

So, specific gravity is dimensionless.

Example 14.
Kinetic energy is given by \(\frac{1}{2}\) mv 2 and potential energy by mgh. Show that both of them have same dimensions.
Answer:
Kinetic energy =\(\frac{1}{2}\) mv²
= [M] ×[LT^-1]^-2=[ML² T^-2]
Potential energy =mgh=[M] ×[LT^-2] ×[L]
= [ML² T^-2]
So, kinetic and potential energies have same dimensions.

Example 15.
Find the dimension of power.
Answer:

Example 16.
In CGS system density of water is 1 g / cc. Find the density of water in SI unit.
Answer:
Density of water =1 g / cc
= 1 g / 1 cm³

∴ Density of water in SI unit is 1000 kgm^-3.

Example 17 :
xpress a light year in megametre
Answer:
Light year is the distance travelled by the light in one year at the speed 3 × 10⁸ m / s.
One megametre =10⁶ m
∴ Distance travelled by light in one year (365 days)

= 94608 × 10⁵ Mm

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 5 work, power and energy offer valuable context and analysis.

WBBSE Class 9 Physical Science Chapter 5 Question Answer – Chapter 5 Work, Power and Energy

Very short answer-type questions

Question 1.
What is the absolute unit of force in SI system?
Answer:
1 joule.

Question 2.
What is power?
Answer:
Work done in unit time is called power.

Question 3.
What is the relation between power and force?
Answer:
Power = force X velocity.

Question 4.
What is energy of a body?
Answer:
Energy is defined as the capacity of a body to perform work.

Question 5.
Is work a scalar or a vector quantity?
Answer:
Work is a scalar quantity.

Question 6.
Write down the absolute and gravitational SI units of work.
Answer:
The absolute SI unit of work : joule.
The gravitational SI unit of work : kg-m.

Question 7.
What is the relation between work and power?
Answer:
Work done (W) = Power P) x Time (t)

Question 8.
What is the gravitational SI unit of power?
Answer:
The gravitational SI unit of power: kg-m/sec

Question 9.
How many kinds of mechanical energy are there?
Answer:
Mechanical energy is of two types :

• Potential energy and
• Kinetic energy.

Question 10.
How is work measured?
Answer:
Work done (W = Force (F) x Displacement (S)

Question 11.
What is the practical unit of work?
Answer:
The practical unit of work is Joule.

Question 12.
Give the expression of kinetic energy ?
Answer:
Kinetic energy =\(\frac{1}{2}\) × mass × (velocity)²

Question 13.
Give the expression of gravitational potential energy ?
Answer:
Gravitational potential energy = mass of the body (m) x acceleration due to gravity (g) x height of the body above the surface of the earth (h).

Question 14.
A man swims in a river against the current water. If he remains fixed with respect to shore, does he work with respect to shore ?
Answer:
Here work done by the man is zero as his displacement with respect to the shore is zero.

Question 15.
Define watt.
Answer:
If an agent does 1 joule of work in 1 second, its power is 1 watt.

Question 16.
Define horse power.
Answer:
The power of work at the rate of 550 ft-lb per second is called one horse power.

Question 17.
Does applied force always perform work ?
Answer:
No work force :

• If there is no displacement of the point of application of the force.
• Again if any force acts perpendicularly to the direction of the displacement.

Question 18.
Is work done due to rotation of the earth round the sun ?
Answer:
A force does no work in a direction perpendicular to its direction of application. Here no work is done.

Question 19.
What do you mean by the rate of work done ?
Answer:
Power.

Question 20.
What is the relation between horse power and watt ?
Answer:
1 horse power = 746 watt.

Question 21.
Give an example of a scalar quantity where three fundamental units are used.
Answer:
Power

Question 22.
Is energy a scalar or a vector quantity ?
Answer:
Energy is a scalar quantity.

Question 23.
What is the SI unit of power ?
Answer:
Watt.

Question 24.
What is the SI unit of energy ?
Answer:
Joule

Question 25.
A heavier body and a lighter body are moving with the same velocity. Which of them can do more work ?
Answer:
Heavier body can do work more.

Question 26.
How much work is done by a man who tries to push the wall of a house but fails to do so.
Answer:
No work is done in this case.

Question 27.
What is the work done by the earth in moving around the Sun ?
Answer:
No work is done.

Question 28.
When is work done by a force negative ?
Answer:
The work done is negative when force acts opposite to the direction of the displacement.

Question 29.
When is work done by a force positive ?
Answer:
The positive work is done when the displacement is in the direction of the force.

Question 30.
identify the kind of energy possessed by a rolling stone.
Answer:
Kinetic energy.

Question 31.
State the law of conservation of energy.
Answer:
Law of conservation of energy: When one form of energy is changed into other forms of energy, the total energy of an isolated system remains the same i.e. the total energy before transformation is equal to the sum of the different energies transformed.

Question 32.
Define Potential energy.
Answer:
Potential energy – Energy acquired by a body by virtue of its position or configuration is called its potential energy and measured by the amount of work the body can do in passing from its present position or configuration to some standard position or configuration.

Short answer type questions 

Question 1.
Define work.
Answer:
Work : Work is said to be done when the point of application of a force moves and is measured by the product of the force and the distance moved in the direction of the force.

Question 2.
How is work measured?
Answer:
Measurement of work:

• If the force F acts on a body and causes a displacement S, then the amount of work done (w) F × S,
• If the direction of displacement makes an angle θ with the direction of the applied force, then component of the force along the direction of displacement will he F cos θ.
  So, work done (w) = F cosθ.S = FS cosθ.

Question 3.
Will work be done by applied forces in all cases ?
Answer:
No work force :

• If there is no displacement of the point of application of the force, then the force is called a no work force.
• Again, if any force acts perpendicularly to the direction of the
  displacement, then the force does not do any work. Because in this case 0 90° and cos 90° = 0.
  The amount of work done = F cosθ.S = F cos 90°.S = 0.

Question 4.
Show with an example that work is done by overcoming some resistive force.
Answer:
When a body lying on a surface is pushed, it moves overcoming the opposing frictional force. Here the body when moves, does some work by overcoming frictional force offered by the surface.

Question 5.
What do you understand by ‘work done by a force’ ? Give one example.
Answer:
Work done by a force : If the displacement of the point of application of the force takes place in the direction of the force then the force is said to have done the work.
Example : When a body is dropped vertically the gravitational force attracts it downward. The displacement of the body takes place in the direction of gravitational force. Here work is done by the gravitational force.

Question 6.
What do you understand by ‘work done against a force’? Give one example.
Answer:
Work done against a force: When the displacement of the point of application of the force takes place in the direction opposite to the applied force, then work is said to be done against the force.
Example: When a body is lifted vertically upward, displacement of the body is in the direction opposite to the gravitational force and work is done against the gravitational force.

Question 7.
What is no-work force ? Give an example.
Answer:
No-work force: When a force acts perpendicular to the direction of displacement of a particle, then the force does not do any work and is known as no-work force.
Example: A porter while carrying luggage does no work, since, he moves horizontally and perpendicular to the force of gravity. The porter, of course, did work at the beginning while he lifted the luggage against the gravity.

Question 8.
State an unbalanced force and indicate the direction and the work it does.
Answer:
Two unequal oppositely directed forces acting on a body create an unbalanced force which is equal to the difference of the forces. The unbalanced force causes displacement and work on the body in the direction of the greater force.

Question 9.
Deduce the relation between joule and erg.
Answer:
Relation between joule and erg :
1 joule = 1 Newton × 1 meter
= 10⁵ dyne × 100 cm
= (10⁵ × 10²) dyne-cm
= 10⁷ erg
∴ 1 joule = 10⁷ erg

Question 10.
Are watt and watt-hour same physical quantities ? Explain.
Answer:
Explanation: Watt and watt-hour are not same physical quantities. Watt which means 1 joule/sec is the unit of power, i.e. work done per unit time, but watt-hour (work/time x time) which equals to work or energy. 1 watt-hour is the work performed by a machine or an agent which works at the rate of 1 joule per second.

Question 11.
Establish a relation between power, force and velocity.
Answer:
Relation between power, force and velocity
\(\text { Power }=\frac{\text { work }}{\text { time }}\)

∴ Power = Force × velocity

Question 12.
If an agent does large amount of work, does it always mean that the agent possesses large power ?
Answer:
Explanation: Only the magnitude of work performed does not clearly indicate the power of a body, if the time in which the work performed is mentioned. It gives the idea of power clearly.

A large quantity of work done, but in a long time, results to a small power but a smaller quantity of work done in a very short period of time results to a greater power.

Question 13.
Express kinetic energy of a body in terms of momentum and velocity.
Answer:
Kinetic energy : Kinetic energy is the energy which a body possesses by reason of its motion.
Kinetic energy (KE) – \(\frac{1}{2}\) × m × υ²
(m = mass of the body
υ = velocity of the moving body)
∴ KE = \(\frac{1}{2}\)m×υ×υ²
(m = mass of the body
υ = velocity of the moving body)
∴ KE = \(\frac{1}{2}\)mυ.υ
\(\frac{1}{2}\)p.υ
(p = mυ = momentum of the body)
So, Kinetic energy = \(\frac{1}{2}\) × momentum × velocity

Question 14.
A particle starting from rest requires kinetic energy ‘E’ in time ‘f and its momentum is ‘p’. Find Its acceleration In terms of E, p and t
Answer:
Deduction: Let acceleration be f, velocity acquired in time t be y and mass of the body be m.

Question 15.
Find the kinetic energy (KE) of a particle of mass ‘m’ gram and momentum ‘p’ g-cm/sec in terms of ‘m’ and ‘p’.
Answer:
Deduction: Let the particle moves with velocity υ cm/sec.

This is the required relation.

Question 16.
Momentum of a heavy body is equal to that of a light body. Which of the two possesses greater kinetic energy?
Answer:
Explanation: Let the mass of the heavy body be M and its velocity υ. The corresponding quantities of the light body are
and ‘υ’.
So, by the given condition, MV = mυ. Since, M is greater than ‘m’,
so ‘υ’ must be proportionally greater than V, for keeping equality between MV and mυ.
Now, if mυ be multiplied with the larger quantity υ and MV with smaller quantity V, the first product mυ.υ will be larger than the second product MV V.
Hence, m.υ.υ. > MV.V.
or, \(\text { or, } \frac{1}{2} m v^2>\frac{1}{2} M V^2\)
i.e. kinetic energy of the light body> kinetic energy of the heavy body.

Question 17.
Two bodies of unequal masses have the same kinetic energy. How do their momenta compare in terms of their masses?
Answer:
Explanation: Let masses of the bodies be m₁, m₂ and their corresponding velocities be υ₁ and υ₂.

Thus, momenta of the two bodies are directly proportional to the square root of their corresponding masses.

Question 18.
A heavy body and a light body have equal kinetic energy, which of them possesses greater momentum?
Answer:
Explanation: Let the mass of a heavier body be m₁ and that of the lighter body be m₂ also let the velocity of the heavier body be y₁ and that of the lighter body be y₂.

So, momentum of the heavier body (m₁υ₁) is greater than the momentum of the lighter body (m₂υ₂)

Question 19.
A man carrying a bag stands in an ascending lift. Explain
(i) does the man do work on the bag?
(ii) is there any change of energy occuring in the bag?
Answer:
Explanation:
(i) Since the weight does not displace with respect to the man, so the man does no work on it.
(ii) The height of the weight from ground gradually increases, so, its potential energy increases. Also, the weight moves upward with the same velocity as that of the lift, so it has kinetic energy.

Question 20.
What is potential energy? Write down a mathematical expression for potential energy of a certain mass raised to a certain height above ground.
Answer:
Potential energy: Potential energy is the energy which a body possesses by virtue of its position or configuration.
Calculation: Weight of the body of mass ‘m’ is mg, where ‘g’ is acceleration due to gravity at the place. To raise the body, an upward force mg has to be applied on it. If it is raised ‘h’ distance above,
work done = mg x h [Force x displacement]
∴ work done = mgh
This work remains stored in the body as potential energy.

Question 21.
Is work done when the Earth moves round the Sun ?
Answer:
Explanation: The centripetal force (the force with which the sun attracts the earth) of the Sun on the Earth does no work, since direction of displacement of the Earth at every point of its orbit is perpendicular to it. So, it is a no-work force.

Question 22.
In a tug of war one team wins over the other. Which team does work and how ?
Answer:
Explanation: In a tug of war the winning team works against the force applied by the losing team who are displaced through some distance.

Question 23.
A moving object suddenly comes to rest. Which energy converts to which other in this case ?
Answer:
In moving condition the body possesses kinetic energy. When it stops, the kinetic energy is expended in doing mechanical work against the opposing forces which stopped the body.

Question 24.
Which energy converts to which other when the spring in a toy is compressed ?
Answer:
Explanation: While the spring of a toy is brought to a compressed condition mechanical energy is expended. This energy remains stored in the spring as potential energy created due to change of configuration of different parts of the spring.

Question 25.
A man carrying a briefcase in hand walks on flat horizontal surface. Does he do work ?
Answer:
Reason: The man does not do work on the briefcase. Because, as the man holds the briefcase in hand, he exerts a force on it vertically upward but the briefcase is made to move horizontally. So, the force and its point of application are perpendicular to each other, i.e. no work is done.

Board answer type question :

Question 1.
What is the comparison of potential and kinetic energy?
Answer:
Comparison of potential and kinetic energy :

Question 2.
What is the comparison of energy and power?
Answer:
Comparison of energy and power:

Energy	Power
(i) The capacity of a body or person to do work is its energy.	(i) Power is the rate of doing work.
(ii) Energy of two persons may be same, but their powers may be different.	(ii) The quantity of energy of two persons may be different, although they have same power.
(iii) Power is quantity dependent on time.	(iii) Energy is not related to time.
(iv) It is a scalar quantity.	(iv) It is a scalar quantity.

Question 3.
In nature, any system tends to be in a position where it possesses minimum energy. Explain with an example.
Answer:
Explanation: In nature, a system tends to be in such a position or state so that it possesses minimum potential or kinetic energy. For this reason, a body possessing some potential energy tends to shift to other position or state to minimise its energy.
Example: A lifted boy seeks the ground level where potential energy of any body is zero with respect to that level.

Question 4.
A body remains at rest at a certain height. State the type or types of energy the body possesses in the following steps :
(i) When the body is at rest at top of the height.
(ii) The body falls through some distance.
(iii) Just before it touches ground.
(iv) it touches ground and becomes stationary.
Answer:
Explanation : A body of mass ‘m’ is at rest above the ground at a height ‘h’. We notice the change of its mechanical energies i.e. kinetic and potential energies at the following stages.

(i) Just before it falls. Here, the body has the potential energy ‘mgh’, and since it is at rest, its kinetic energy is zero. So, at this stage total energy of the body is mgh.

(ii) The body descends through ‘a’, where, a < h.
The height of the body above ground is (h – a), so its potential energy is mg (h – α). The potential energy thus decreases by mga. While the body descends, it acquires some velocity, υ(say). So its kinetic energy is \(\frac{1}{2}\) mυ² which can be proved equal to mga. Thus decrease of potential energy equals gain in kinetic energy.
So, the total energy
= mg (h – α) + mga
= mgh – mgα + mgα = mgh

(iii) The body is just above the ground before touching it. Here ‘h’ is almost zero, so the potential energy is negligible. The body possesses only kinetic energy which may be proved equal to mgh. Thus, at all stages, the total energy of the body, before it touches ground, is constant and equal to mgh.

(iv) When the body touches ground ‘h’ is zero, so potential energy is zero. Since, it comes to rest, its kinetic energy is also zero. The energy of the body dissipates in environment as heat energy and sound energy.

Question 5.
Can a body possess energy without momentum ? is the reverse possible ?
Answer:
Explanation: A body remaining motionless at a certain position other than its normal position on the surface of the earth may have potential energy but it is without momentum. The body also has no kinetic energy.

In the reverse case, if the body has momentum it must have velocity and thus kinetic energy, hence momentum without kinetic energy is not possible. Of course, the body may have momentum without potential energy, if it confines its motion on the surface of the earth.

Numerical Problems

Working formula

(i) Work done = Force x displacement of the point of application.

(ii) Power (p) = \(\frac{\text { work done }(w)}{\text { time taken }(t)}\)

(iii) Kinetic energy (KE) = \(\frac{1}{2}\) × mass of the body (m) × (velocity of the body)² (y)²

(iv) Potential energy
(PE) – mass of the body (m) × acceleration due to gravity (g) × height of the body above the surface of the earth (h)

(iv) 1 Horse power = 746 watt

(vi) 1 kilowatt = 10³ watt

(vii) 1 mega watt = 10⁵ watt.

Problem 1.
A rifle bullet of mass 15g travelling at a speed of 300 meter/sec. Find the kinetic energy of the bullet
Answer:
m = 15, g = 0.015kg
y = 300 m/sec
KE =?
kinetic energy = \(\frac{1}{2}\) × mass ×(velocity)²
= \(\frac{1}{2}\) × m × ν²
= \(\frac{1}{2}\) × 0.015 x (300)²
= 675 joule.

Problem 2.
The power of a motor car is 10 kW. What is the amount of work done by the car in 1 hr?
Answer:
1 hr = 60 × 60 = 3600 sec
1 kW = 10³W
Power of the car = 10 kW = 10 × 10³ J/sec
∴ work done by the car in 1 hr
= 10 × 10³ x 3600J
= 36 × 10⁷J

Problem 3.
If a pump lifts 500 liter of water per hour to a height of 30m. Then find the work done by the pump and its power in watt. [mass of 1 liter of water = 1 kg]
Answer:
Work done by the pump
= weight of water x displacement of water
= 5000 × 1 × 98 × 30J
= 14.7 × 10⁵J
Power of the pump = \(\frac{\text { Work done }}{\text { time }}=\frac{14.7 \times 10^5 \mathrm{~J}}{60 \times 60 \mathrm{sec}}\)
= 408W

Problem 4.
If a boy weighing 40 kg runs up a flight of stairs to a height of 10 m against the gravitational pull on him, i.e. his weight, so
Answer:
Work done = 40 × 98 × 10 = 3920 J
Power of the boy = \(\frac{\text { Work done }}{\text { Time }}=\frac{3920}{15}\) = 261.33W

Problem 5.
If 150 joule work is done for the displacement of 15 m of a body. Calculate the amount of force applied here.
Answer:
Work is done here = 150 joule
displacement = 50 m
we known,Applied force  = \(\frac{\text { Work done }}{\text { displacement }}\)
= \(\frac{150}{50}\) = 3 Newton

Problem 6 :
If 10 Newton force is applied on a body then 5 m displacement of the body occurs towards the direction of the force. What will be the work done?
Answer:
We know,
Work done = Force x displacement
= 10 × 5 Newton-meter
= 50 joule

Problem 7.
If 100 g mass of a body is dropped from 80 cm high place then calculate the amount of potential energy is lost during this
process.
Answer:
m = 100 g = 0.1 kg
g = 98 m/sec²
h = 80 cm = 0.8 m
Amount of potential energy lost = m × g × h
= 0.1 × 9.8 × 0.8 joule
= 0.784 joule

Problem 8.
One body has mass 50 g and velocity 10 cm/sec. The other body has mass 20 g and velocity 20 cm/sec. Compare their velocities and kinetic energies.
Answer:
We know,
Momentum of a body = mass of the body × velocity
Here, velocity of first body : velocity of second body
=(50×10):(20×20) = 5 : 4
We also know,
Kinetic energy = \(\frac{1}{2}\) × mass × (velocity)²
Here, kinetic energy of first body: kinetic energy of second body
= \(\frac{1}{2}\)× 50 × (10)² : \(\frac{1}{2}\) × 20 × (20)²
∴ The ratio of velocities of two bodies = 5 : 4
The ratio of kinetic energies of two bodies 5 : 8

Problem 9.
A rifle bullet of mass 50 g travelling at a speed of 200 rn/sec. Find the kinetic energy of the bullet.
Answer:
m= 50g g = 0.05kg
υ = 200 m/sec
KE=?
Kinetic energy = \(\frac{1}{2}\) × mass × (velocity)²
\(\frac{1}{2}\) × m × (velocity)²
= \(\frac{1}{2}\) × m x ν²
= \(\frac{1}{2}\) 005 × (200)²
= 10³ joule

Problem 10.
The power of a machine is 40 W. What is the amount of work done by the machine in 30 sec?
Answer:
Power of the machine = 40 W
= 40 joule/sec
∴ Work done by the machine (40 × 30) joule
= 1200 joule

Problem 11.
A car and a truck have the same speed of 30ms. If their masses are in the ratio of 1: 3, find the ratio of their kinetic energies.
Answer:
Given V₁ = V₂ 30 ms^-1
m₁ = m₂ = 1 : 3

Problem 12.
A particle of mass ‘m’ at rest is acted upon by a force ‘p’ for a time ‘t’. Show that the particle acquires the kinetic energy after the time ‘t’.
Answer:
It is known from Newton’s second law of motion that, force (p) = mass (m) × acceleration (f)

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 4.6 Water offer valuable context and analysis.

WBBSE Class 9 Physical Science Chapter 4.6 Question Answer – Water

Very short answer type questions

Question 1.
On which basis water is called hard and soft?
Answer:
Water is classified in hard and soft according to its ability of forming lather with soap.

Question 2.
Chemical activity of which is greater — potassium and magnesium ?
Answer:
Potassium has greater chemical activity than magnesium.

Question 3.
What type of water is used in a boiler ?
Answer:
Soft water

Question 4.
Which gas is produced when calcium reacts with water ?
Answer:
Hydrogen.

Question 5.
Which one has greater affinity towards water — Zn or Hg ?
Answer:
Zn.

Question 6.
In the activity series of the metals, which one has upper position – Ai or Au ?
Answer:
Al.

Question 7.
What is temporary hardness of water ?
Answer:
Temporary hardness of water is caused by the presence of soluble bicarbonates of calcium, magnesium and iron in it.

Question 8.
What is the cause of hardness of water ?
Answer:
Hardness of water is caused due to dissolved bicarbonate, chloride, sulphate salts of Ca, Mg and Fe in water.

Question 9.
State two causes of water crisis.
Answer:
Causes of water crisis :

•  Misuse of water
• Water pollution.

Question 10.
What are the products obtained from the reaction of sodium and water ?
Answer:
The two products are Sodium hydroxide and hydrogen.

Question 11.
What is the cause of black foot disease ?
Answer:
Black foot disease is caused by arsenic poison.

Question 12.
What is fluorosis disease ?
Answer:
Excess fluoride in water is responsible for fluorosis disease.

Question 13.
What are the types of hardness of water ?
Answer:
Two types of hardness of water :

• Temporary hardness
• Permanent hardness.

Question 14.
Which one is more pure ……….. deionised water or distilled water ?
Answer:
Distilled water.

Question 15.
What is electrochemical series ?
Answer:
Electrochemical series is arranged the position of metals according to their standard oxidation potential values.

Question 16.
Which metal reacts with water at ordinary temperature ?
Answer:
Potassium.

Question 17.
What is the process by which both types of hardness are removed ?
Answer:
Removal of both types of hardness is done by ion-exchange process

Question 18.
Give one cause of water crisis.
Answer:
Wastage of water.

Question 19.
What amount of potable water is present in total water of earth ?
Answer:
0.3%.

Question 20.
Which metalloid is the cause of water pollution ?
Answer:
Arsenic (As).

Question 21.
What amount of water is present in human body ?
Answer:
70%.

Question 22.
What is the value of specific heat of water ?
Answer:
4200  kg^-1 K^-1.

Question 23.
What is the latent heat of fusion of ice ?
Answer:
80 cal g^-1

Question 24.
What is the latent heat of vaporisation of water ?
Answer:
540 cal g^-1

Question 25.
What is potable water ?
Answer:
Potable water — Natural water free from suspended impurities and dissolved poisonous impurities, germs, bacteria, etc., so that it can be safely used for consumption of human beings and animals, is called potable water.

Question 26.
What is the method by which coliform count is made ?
Answer:
Most probable number index method (MPNI).

Question 27.
Dissolved oxygen is measured in which unit ?
Answer:
mg L^-1

Question 28.
What is Algal bloom ?
Answer:
An Algal bloom is a rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system.

Question 29.
Which plant can reduce the amount of fluoride in drinking water ?
Answer:
Tulsi plant leaf can reduce the amount of fluoride in drinking water.

Question 30.
What is the function of AIAA ?
Answer:
Alumina impregnated activated alumina (AIAA) is an important adsorbent for removal of fluoride.

Short answer type questions 

Question 1.
What are the sources of natural water ?
Answer:
Sources of natural water: Natural water is available from various sources, such as seas, rivers, lakes or ponds, wells, springs and rain.

Question 2.
What is soft water ?
Answer:
Soft water: The class of water that easily forms lather or foam with soap is called soft water.

Question 3.
What is hard water ?
Answer:
Hard water: The class of water that does not easily form lather with soap, or does so after much wastage of soap is called hard water.

Question 4.
What is the cause of hardness ?
Answer:
Causes of hardness of water: The hardness of water is due to the presence of dissolved salts of metals (except Na, K) particularly calcium, magnesium and iron.

Question 5.
What is soap ?
Answer:
Soap : Soap is a salt of sodium or potassium of giant acid. Fatty acids are normally stearic acid (C₁₇H₃₅COOH), palmitic acid (C₁₅H₃₁COOH), oleic acid (C₁₅H₃₃COOH).

Question 6.
What is meant by temporary hardness of water ?
Answer:
Temporary hardness of water: Temporary hardness is caused by the presence of soluble bicarbonates of calcium, magnesium and iron in water.

Question 7.
What is meant by permanent hardness of water ?
Answer:
Permanent hardness of water: Permanent hardness is caused by the presence of chloride and sulphate salts of calcium, magnesium and iron in water.

Question 8.
What is meant by removal of hardness of water ?
Answer:
Removal of hardness of water: Hardness is due to the dissolved calcium, magnesium and iron salts. Now if these soluble salts are converted to insoluble salts by any process, then hardness of water is removed.

Question 9.
What is permutit ?
Answer:
Permutit : Permutit is a trade name for artificially prepared zeolite which is sodium aluminosilicate (Na₂O, Al₂O₃, 3SiO₂ ,2H₂O). Permutit is used to remove the hardness (both types, temporary and permanent).

Question 10.
What is deionised water ?
Answer:
Deionised water : The water that contains no cation other than H⁺ ion and anion other than OH^– ion is called deionised water. Deionised water is prepared only when hard water is allowed to pass through both cation ion-exchange and anion exchange resins.

Uses :

• In chemical laboratory for qualitative as well as quantitative analysis of presence of ions in a substance, other than OH^– and H⁺ ions.
• In batteries having lead electrodes.

Question 11.
Discuss the action of water on sodium and calcium metals ?
Answer:
Action of sodium and calcium on water: Sodium and calcium react promptly with water at ordinary temperature with fire and sound. At the end of the reaction metallic hydroxides and hydrogen are formed :
2Na + 2H₂O = 2NaOH + H₂ ↑
Ca + 2H₂O = Ca(OH)₂ + H₂ ↑

Question 12.
What is water pollution ?
Answer:
Water pollution: Some soluble and insoluble matters in water which produce some harmful effect to men and aquatic animals are called water pollution.

Question 13.
What is black foot disease ?
Answer:
Black foot disease: Ground water sometimes is contaminated with arsenic acid and arsenius acid as a deprotonated form. Consumption of these arsenic contaminated water for a long period has some poisonous effects in the form of black spots on palms and feets, skin become rough. The disease caused by arsenic poison is called black foot disease.

Question 14.
What is Fluorosis ?
Answer:
Fluorosis: Sea-water, river water contain fluoride from natural source. Excess fluoride in water is responsible for fluorosis disease which is a cause of dental decay and bone decay.

Question 15.
Write down two processes of getting arsenic free water.
Answer:
Two processes of getting arsenic free water :

• Arsenic is removed from water if water is allowed to pass through a bag made of cloth containing alumina or ferric hydroxide replacing the candle used in the filter. Arsenic or arsenious ions are absorbed in the alumina bed or ferric hydroxide bed and thus are removed.
• Take 20 liter of water, i- amount of tea-spoonful of bleaching powder and the same amount of ferrous sulphate in a bucket. After stirring well, arsenic is removed.

Question 16.
What is water crisis ?
Answer:
Water crisis: Within 2010 AD we are to face a critical situation for potable water. This is because of population is gradually increasing though storage of sweet water is fixed. This crisis can be overcome by avoiding the wastage of water and by judicious use of available water.

Collection of rain water and its uses for certain purposes like gardening, car-washing, floor-washing and even bathing can also mitigate water crisis. Scientists in different countries are engaged in finding a solution how saline water in sea be desalinated.

Question 17.
How is harvesting of water done for agriculture ?
Answer:
Harvesting of water for agriculture : In India and also in some other parts of world excessive rain water is stored in underground reservoirs. Such subsurface stores of water is freed from huge loss of water by evaporation as it happens in surface reservoirs. This stored water may be used for agricultural works

Board Answer Type Question 

Question 1.
Describe a method of removal of temporary hardness of water.
Answer:
Removal of temporary hardness by boiling: Temporary hardness of water is due to the presence of soluble bicarbonates of calcium, magnesium and iron in water. It is removed by boiling when soluble bicarbonate precipitate as insoluble carbonates and carbon dioxide gas evolves. On filtration soft water is obtained.

Question 2.
Describe the process by which both the temporary and the permanent hardness of water are removed.
Answer:
Removal of both temporary and permanent hardness by ion-exchange resin process :

Principle: Cations present as soluble salts in hard water are exchanged with the cations present in suitable resins producing insoluble salts containing the resin. After that step acids are produced, anions of which are exchanged with the anion present in other type of suitable resins.
Nature and function of resin : Resins are artificially prepared organic polymers widely used to remove hardness of water. These are of two types :

(a) Cation-exchanger resins carrying radicals like, -SO₃H, – COOH contain exchangeable W ions. These cations can be exchanged for equivalent positive ions like Ca₂, Mg₂, etc. present in soluble salt in water.

(b) Anion-exchanger resins carry – NHOH, NH₃OH containing exchangeable OH ions.

Process:
(a) On passing hard water through a chamber containing the cation-exchanger resin, exchange of Ca₂ and Mg₂ with the W of the  – COOH or – SO₃H group of the resin takes place.

The reactions are :

Now the water obtained after passing through the cation exchanger is acidic.

(b) Water after passing through cation exchange resin is now allowed to flow over anion exchange resin (expressed as RNH) due to exchange of anion, resin gets precipitated. Water thus obtained is completely deionised.

After these two steps, hardness of water is completely removed,the water contains no cation other than H⁺ ions and no anion other than OH^– ions.

Useful-exchangers :
(a) Cation-exchangers :

•  Amberlite IR-120
• Dowex-50
• Zeocarb 225

(b) Anion-exchangers :

• Amberlite IRA-400
• Dowex I
• Dowex II

Regeneration of resins: Cation exchange resin is regenerated by passing dilute H₂SO₄ and anion exchange resin is generated by using dilute solution of NaOH solution.

Question 3.
Why is water called versatile solvent?
Answer:
Water is a versatile solvent: Water molecule is amphiprotic, i.e. it behaves both as an ionic compound and a covalent compound. As a result, water can dissolve both ionic and covalent compound. Thus, on the one hand, acids, bases and salts (ionic compounds) easily dissolve in water and on the other hand, it can dissolve covalent compounds like alcohol, (methanol, ethanol), acetone, glycerol, sugar, urea, etc.
For this reason water is called versatile solvent.

Question 4.
How is delonised water prepared?
Answer:
Preparation of deionised water : Water is treated with some resins which contain carboxylic acid radical  – COOH or  – SO₃H
(suiphonic acid radical). The cations, Ca²⁺, Na⁺, Fe²⁺, etc. present in water are removed on exchange of these with H⁺ ions of – COOH or – SO₃H of resins.
If the resin is represented as RH, where R is the resin polymer,
H is hydrogen of acid radicals present in the resin.
CaCl₂ + 2RH =  R₂Ca↓ + 2HCl
NaCl + RH = RNa + HCl
After removal of cations, water contains some acid radicals like, HCl. To remove the acid radicals, water is treated with another type resin of basic nature. In this way, the water is freed from all types of ions, except H’ and OH- ions and this is deionised water.

Question 5.
(a) What is fluoride ?
(b) Name one harmful fluoride and state some of its bad effects on human.
(c) State some injurious effects of presence of fluoride in water.
Answer:
(a) Fluoride is meant by any binary compound of fluorine.
(b) The most harmful compound of fluorine is hydrogen fluoride (HF). It violently attacks skin, forming sores. Due to inhalation of vapour of this fluoride, loss of voice takes place and finally death results.
(c) Presence of traces of some fluorides in water is very dangerous.
Some of the injurious effects of fluoride are given below.

• Loss of calcium from bones causing skeletal weakness.
• Carries and discolouration of teeth.
• Bending of the knees of sides, known as, Genu Valgum.

Question 6.
(a) When the water in a soil layer may be considered arsenic-polluted?
(b) State two harmful effects of drinking water that contains arsenic beyond permissible limit.
(c) State one arrangement for supply or arsenic-free water.
Answer:
(a) According to the World Health Organisation (WHO), the permissible limit of arsenic in drinking water is 0.05 mg/L. So, the water in a soil layer that contains more than this specified quantity of arsenic is polluted.

(b) Harmful effects of arsenic : On drinking water containing arsenic beyond the permissible limit, diseases like skin pigmentation, organ failure may occur.

(c) Supply of arsenic-free water :

• Arsenic is removed from water, if water is allowed to pass through a bag made of cloth containing alumina or ferric hydroxide replacing the candle used in the filter. Arsenic or arsenious ions are absorbed in the alumina bed or ferric hydroxide bed and thus are removed.
• Take 20 liter of water, \(\frac{1}{4}\)th amount of tea-spoonful bleaching powder and the same amount of ferrous sulphate in a bucket. After stirring well, arsenic is removed.

Question 7.
Discuss the difference of activity of the metals with water on the basis of the position of the metals in Electrochemical series.
Answer:
Difference of activity of the metals in reaction with water:
(a) A metal with highest electron releasing ability is placed in the top most position. Metals placed in the electro-chemical series react with water according to their positions in the series.

(b) Chemical activity of metal entirely depends on their position in the electro-chemical series. Metal which occupies higher position than any other metal has greater value of

• Electropositive character
• Reducing capacity
• Chemical reactivity.

Question 8.
What is Permutit process?
Answer:
Permutit process: Both types of hardness can be removed cheaply in this process. Permutit is a trade name for artificially prepared zeolite which is sothum aluminosiicate (Na₂O, Al₂O₃, 3SiO₂, 2H₂O). It is a natural hydrated large-sized crystalline substance. A bed of granules of permutit is kept in a porous shelf slightly upper middle of a big drum made of iron.

Other porous shelves staged with stone and sand are placed above and below the shelf containing bed of permutit. When hard water kept in a tank is allowed to pass through the bed of permutit due to exchange of ions the hardness of water is removed and soft water is obtained. Let the ion-exchange material permutit be formulated as Na₂Ze, where Ze is the zeolite radical.

After the use of permutit for sometime when the permutit gets exhausted and loses its activity. It is regenerated by passing an aqueous solution of sodium chloride by which permutit gets back its exchange capacity.
CaZe + 2NaCl = Na₂Ze + CaCl₂
MgZe + 2NaCl Na₂Ze + MgCl₂
Permutit water-softening plants are used in private houses and factories and also for softening public water supplies of towns.

Question 9.
Discuss water pollution. How it is caused ? What are the dangers of water pollution ?
Answer:
Water pollution : Some soluble and insoluble matters in water which produce some harmful effect to men and aquatic animals are called water pollution.

Cause of water pollution : Pollution of water may be natural or man-made. Natural water coming from underground sources like tube-wells, wells, etc. and that flowing over earth’s surface often give rise to natural pollution due to presence of fluorides of alkali metals and arsenic compounds.

Man-made water pollution arises from the indiscriminate use of fertilizers, insecticides, pesticides in agriculture, and by the dumping of chemical effluents like metal salts, acids, alkalis, etc. from factories into the river or lakes. The washing of clothes and cooking utensils, bathing men and animals make water of ponds and lakes polluted.

Apart from these, the sub-soil water are contaminated with worms, bacilli amoeba and several other pathogenic bacteria and virus.

Dangers of water pollution :

• Appreciable amounts of fluorides in water give rise to gum diseases, decay of the enamel of teeth and also deseases of bones.
• Presence of considerable amount of arsenic compounds in water causes ailments of the digestive system and severe skin diseases. Consumption of arsenic contaminated water for a long period has some poisonous effects in the form of black spots on palms and feets. Black foot disease is due to arsenic poison.

West Bengal Board Class 9 Physical Science Notes WBBSE

1. Measurement Class 9 WBBSE Notes
2. Force and Motion Class 9 WBBSE Notes
3. Matter: Structure and Properties Class 9 WBBSE Notes
4. Matter: Atomic Structure, Physical and Chemical Properties of Matter Class 9 WBBSE Notes
5. Work, Power and Energy Class 9 WBBSE Notes
6. Heat Class 9 WBBSE Notes
7. Sound Class 9 WBBSE Notes

WBBSE Class 9 Solutions

Well structured WBBSE Class 9 Physical Science MCQ Questions Chapter 4.6 Water can serve as a valuable review tool before exams.

Water Class 9 WBBSE MCQ Questions

Multiple Choice Questions :

Question 1.
The salt which is the cause of hardness of water :
(i) NH₄ Cl
(ii) NaNO₃
(iii) MgSO₄
(iv) K₂ SO₄
Answer:
(iii) MgSO₄

Question 2.
Soap is a salt of :
(i) Acetic acid
(ii) Palmitic acid
(iii) Formic acid
(iv) Nitric acid.
Answer:
(ii) Palmitic acid

Question 3.
Which of the following metals does not react with cold and hot water :
(i) Fe
(ii) K
(iii) Ni
(iv) Ag
Answer:
(iv) Ag

Question 4.
Which of the following is soft water:
(i) rain water
(ii) river water
(iii) tube well water
(iv) sea water
Answer:
(i) rain water

Question 5.
The salt which is the cause of temporary hardness :
(i) NaHCO₃
(ii) NaCl
(iii) Ca(HCO₃)₂
(iv) MgSO₄
Answer:
(iii) Ca(HCO₃)₂

Question 6.
Which of the following metals reacts easily with water at ordinary temperature :
(i) Ca
(ii) Zn
(iii) Cu
(iv) Ag
Answer:
(i) Ca

Question 7.
Tube well water becomes brown due to :
(i) Ca(HCO₃)₂
(ii) Fe(HCO₃)₂
(iii) MgSO₄
(iv) MgCl₂
Answer:
(ii) Fe(HCO₃)₂

Question 8.
D₂ O reacts with CaC₂ to form :
(i) C₂ D₂
(ii) CaD₂
(iii) Ca₂ D₂ O
(iv) CD₂
Answer:
(i) C₂ D₂

Question 9.
Permutit is regenerated by passing :
(i) 10% NaOH solution
(ii) 10% NH₄ Cl solution
(iii) 10% NaCl solution
(iv) none of these
Answer:
(iii) 10% NaCl solution.

Question 10.
If 1 lit. water contains 0.001 mol MgSO₄. What will be the hardness of water?
(i) 100 ppm
(ii) 200 ppm
(iii) 500 ppm
(iv) 150 ppm
Answer:
500 ppm

Question 11.
Water has high boiling point because :
(i) specific heat of water is high
(ii) heat of dissociation is high
(iii) presence of hydrogen bonding
(iv) molecules weight is low
Answer:
Presence of hydrogen bonding.

Question 12.
Anion exchange resin is regenerated by passing :
(i) dilute NaCl solution
(ii) dilute H₂ SO₄ solution
(iii) dilute HNO₃ solution
(iv) dilute NaOH solution
Answer:
dilute NaOH solution

Question 13.
When hard water is passed through zeolite, the Na (present in zeolite) is exchanged by :
(i) OH^–
(ii) Ca²⁺
(iii) \(\mathrm{SO}_4^{2-}\)
(iv) H⁺
Answer:
Ca²⁺

Question 14.
Water is oxidised to oxygen by :
(i) ClO₂
(ii) KMnO₄
(iii) H₂ O₂
(iv) F₂
Answer:
F₂

Question 15.
Sodium promptly reacts with water at ordinary temperature to form sodium hydroxide and what ?
(i) oxygen
(ii) nitrogen
(iii) chlorine
(iv) hydrogen
Answer:
hydrogen

Question 16.
Which state of water provides medium of life ?
(i) Solid
(ii) liquid
(iii) vapour
(iv) gas
Answer:
liquid.

Question 17.
Which property of water controls environmental temperature ?
(i) specific heat capacity
(ii) Latent heat of vaporisation
(iii) density
(iv) surface tension
Answer:
Specific heat capacity

Question 18.
Air obtained from dissolved air in water contain oxygen and nitrogen respectively in what percentage?
(i) 21% O₂, 78% N₂
(ii) 17% O₂, 83% N₂
(iii) 33% O₂, 66% N₂
(iv) 66% O₂, 33% N₂
Answer:
33% O₂, 66% N₂

Question 19.
What is the prescribed limits of pH of potable water?
(i) 4.5
(ii) 6.5-8.5
(iii) 1.5-4.5
(iv) 8.5-9.5
Answer:
6.5-8.5

Question 20.
Which of the following ions responsible for hardness of water?
(i) Fe²⁺
(ii) Na⁺
(iii) Cr³⁺
(iv) Mn²⁺
Answer:
Fe²⁺

Fill in the blanks :

1. _______ reacts with water at ordinary temperature.
Answer:
Calcium

2. Soft water readily forms lather with _______.
Answer:
soap

3. Water of sea, river are examples of _______ water.
Answer:
hard

4. By using _______ water thermal plants can cut water requirements by about 98%.
Answer:
recycled

5. Depletion of oxygen dissolved in water due to enhanced growth of algae is called _______.
Answer:
eutrophication

6. Elements having greater tendency to gain electrons from their solution are called _______ elements.
Answer:
electronegative

7. Deionised water contains only H⁺ cation and _______ anion.
Answer:
OH^–

8. A more reactive metal displaces a _________ reactive metal from the latter’s solution.
Answer:
less

9. Cation-exchanger resins contain COOH, _______ radicals.
Answer:
SO₃ H

10. Temporary hardness of water is due to presence of soluble _______
Answer:
bicarbonates

11. Permanent hardness of water is due to presence of dissolved chloride and _______ salts of Ca and Mg.
Answer:
sulphate

12. The class of water that does not easily form _______ with soap is called hard water.
Answer:
lather

13. The class of water that easily form lather with soap is called ______ water.
Answer:
soft

14. Water is a _______ solvent.
Answer:
versatile

15. Removal of both temporary and permanent hardness is done by ______ process.
Answer:
ion exchange

16. Removal of temporary hardness of water is done by _______.
Answer:
boiling

17. De-ionised water is made by _______.
Answer:
de-ioniser

18. Among river water, rain water and tubewell water, _______ is soft water.
Answer:
rain water

19. Cu does not react with _______ at ordinary temperature.
Answer:
water

20. The 97 % water of the total amount of earth’s water is called _______ water.
Answer:
underground

21. _______ is the chemical name of zeolite.
Answer:
Sodium aluminosilicate (Na₂ O, Al₂ O₃, 3 SiO₂, 2 H₂ O)

22. The permissible limit of arsenic in drinking water is _______.
Answer:
0.05 mg / L

23. Amberlite IR-120 is a useful _______ exchanger.
Answer:
cation

24. RSO₃ H is a ______ exchanger resin.
Answer:
cation

25. If pressure on the surface of water is increased, its boiling point and freezing point ______.
Answer:
increases, decreases.

26. Temporary hard water contains ______ salt of calcium, magnesium and iron.
Answer:
hydrogen carbonate

27. RNH₃⁺ OH – is ______ exchanger resin.
Answer:
anion