# WBBSE Class 9 Physical Science Solutions Chapter 2 Force and Motion

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

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

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

Question 3.
In which part of the earth the weight of a body is maximum ?
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 ?
Spring balance.

Question 5.
What is the ultimate source of all energies ?
The sun. Question 6.
In an electric cell which type of energy changes to which other ?
Chemical energy to electric energy.

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

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

Question 9.
How is tidal energy explored?
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 ?
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 ?
Mass remains same, weight decreases.

Question 12.
Where does a body weigh more, at the equator or at the poles of the Earth ?
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 ?
Mass of a body is measured in common balance.

Question 14.
Write down the relation between mass and weight.
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 ?
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?
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.
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.
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.
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.
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.
In telephone and microphone, sound energy is converted into electrical energy.

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

Question 23.
How is potential energy converted into kinetic energy ?
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.
Nuclear energy is not derived from solar energy.

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

Question 26.
Is wood matter ?
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 ?
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 ?
Newton.

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

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

Question 31.
Give an example of transformation of kinetic energy to potential energy.
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?
All things in satellite will be weightless. Question 33.
Define the term displacement.
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 ?
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.
Rate of change of position of a body with respect to time is known as its speed.

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

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

Question 38.
Define velocity.
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.
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.
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.
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.
To every action there is an equal and opposite reaction.

Question 1.
When a candle burns out, the molten wax weighs less than the candle. What happens to the ‘missing mass’?
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?
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.
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?
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 ?
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 ?
Each form of energy ultimately transforms to heat energy directly or after some intermediate transformations. Question 7.
Define matter.
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 ?
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 ?
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 ?
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 ?
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 ?
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 ?
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 ?
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 ?
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.
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’?
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 ?
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 ?
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 ?
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 ?
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.
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.
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.
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.
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.
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 ?
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 ?
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 ?
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 ?
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.

Question 1.
Why does the value of ‘g’ vary from place to place?
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.
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.
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.
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.
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.
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.
Some examples of transformation of energy : Question 8.
Deduce Newton’s first law of motion from the second law.
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 ?
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 ?
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.
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 ?
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?
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.
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?
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)
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 ?
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 ?
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,
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.
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 )
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.
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?
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 ?
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 = $$\frac{m f}{2 m}=\frac{f}{2}$$