# WBBSE Class 8 Science Solutions Chapter 1.1 Force and Pressure

Detailed explanations in West Bengal Board Class 8 Science Book Solutions Chapter 1.1 Force and Pressure offer valuable context and analysis.

## WBBSE Class 8 Science Chapter 1.1 Question Answer – Force and Pressure

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
A push or pull on an object is called as–
i) push-pull
ii) force
iii) pressure
iv) all of above
ii) force

Question 2.
What is the cause of change in motion or change in the state of motion?-
i) pressure
ii) friction
iii) atmospheric pressure
iv) force
iv) force

Question 3.
A ball rolling on the ground slows down and finally stops. It is because of –
i) force
ii) less force
iii) friction
iv) none of the above
iii) friction

Question 4.
Pressure is defined as-
i) Area/Force on which it acts
ii) Force/Area on which it acts
iii) Volume/Force on which it acts
iv) Force/Volume on which it acts
ii) Force/Area on which it acts

Question 5.
In a tag of war, two teams pulling a rope does not move towards any team, it implies that-
i) An equal force is being applied in the opposite direction
ii) An equal force is being applied in the same direction
iii) No force is applied in any direction
iv) Can not be explained
i) An equal force is being applied in the opposite direction

Question 6.
When two forces, applied on an object, are equal and opposite, then the forces-
i) May move the object
ii) May stop a moving object
iii) May move the object and also cause a change
iv) Do not move the object but may cause a change in its shape
iv) Do not move the object but may cause a change in its shape

Question 7.
Friction produces-
i) light
ii) alpha rays
iii) heat
iv) all of the above
iii) heat

Question 8.
A liquid exerts pressure in directions-
i) upward
ii) downward
iii) sidewards
iv) all
iv) all

Question 9.
The air prèssure on our body is equal to-
i) Atmospheric pressure
ii) sea botton pressure
iii) space pressure
i) Atmospheric pressure

Question 10.
The SI unit of pressure is-
i) N m3
ii) N m2
iii) kg m2
iv) pa m2
ii) N m2

Question 11.
The standard atmospheric pressure is –
i) pressure of a mercury column of height 76 cm
ii) mercury of 10 metre
iii) pressure of 76 mm mercury column
i) pressure of a mercury columin of height 76 cm

Question 12.
The upward force exerted on a body by the fluid in which it is submerged is called the –
i) Immersion
ii) Buoyancy
iii) Weight
ii) Buoyancy

Question 13.
On which of the following pressure of liquid does not depend –
i) depth of liquid
ii) upper surface area of liquid
iii) density of liquid
ii) upper surface area of liquid

Question 14.
Density of saline water is – than purified water.
i) more
ii) less
iii) same
i) more

Question 15.
Density of mercury is –
i) low high
iii) very high
iv) very low
iii) very high

State whether True or False : (1 mark for each question)

1. Presure of saline water is greater than purificd water.
2. Buoyancy is a kind of force.
3. If the weight of an object is greater than buoyancy of that object it gets immerged in water.
4. Pressure on any point of liquid is same.
5. In SI system unit of pressure is Newton/meter 2
6. Density of kersone oil is less than desnity of water.
7. Buoyancy acts towards the weight of an object.
8. The syringe works on a principle of atmospheric prussure.
9. Newton’s first law of motion is alternatively called as law of inertia,
10. Cutting and piercing tools have blunt edeges.
11. Camel can not walk on sand due to its broad feet.
12. Force of friction is greater in case of smeoth surface.
13. Fluids exert pressure.
14. When an object remains stationary on a surface, the object exerts downward force on the surface vertically
15. Pressure and force dscides the direction of flow of a liquid.
1. true
2. false
3. true
4. false
5. true
6. true
7. false
8. true
9. true
10. false
11. false
12. false
13. true
14. true
15. fale

Fill in the blanks : (1 mark for each question)

1. Force = mass of the objeet × ______.
2. SI unit of measuring forte ______.
3. When you are holling a weight of 1 kg in your hand, the force that is applied on your hand is around ______ Newton.
4. We can change the derection of a moving body by applying ______.
5. Litre is the unit of ______.
6. Frictional force works against the ______ of an object.
7. In SI system unit of density is ______.
8. Density of nercury = ______ gram/cubic centimetre.
9) Mass = Density × ______.
10. The force per unit area applied to the surface of a object is known as ______.
1Question 1. The strength of force is usually expressed by its ______.
12. Powder is sprinkled on the carom board to reduce ______.
13. Sliding friction is ______ than the static friction.
14. Friction depends upon ______ of the surface.
15. Water and other liquid exerts ______ when objects move through them.
1. acceleration
2. Netwon
3. 9.8
4. force
5. volume
6. velocity
7. Kg m3
8. 13.6
9. volume
10. pressure
11. magnitude
12. friction
13. smaller
14. nature
15. force of friction.

Match the following : (1 mark for each question)

a) Match the column A with column B.

 Column A Column B a) In CGS system unit of force is i) Buoyancy b) If density incrase presure of liquid ii) is greater than purified water. c) Value of acceleration due to gravity iii) 980 cm / sec2 d) The principle of buoyancy is stated by iv) measuring cylinder e) Upward force exerted on a body is called v) is called acceleration f) Density of saline water vi) 980 cm / sec2 g) Volume of liquid is measured by vii) dyne h) The rate of change of velocity with time viii) scientist Archimedes i) Density of water is ix) increase

a) In CGS system, unit of force is dyne.
b) If density increase pressure of liquied increase.
c) Value of acceleration due to the gravity 980 cm sec2
d) The principle of buoyancy is stated by scientist Archimedes.
e) Upward force exerted on a body is called buoyancy.
f) Density of saline water is greater than purified water.
g) Volume of liquid is measured by measuring cylinder,
h) The rate of change of velocity with time is called acceleration.
i) Density of water is lg cm3.

b) Match the column A with column B.

 Column A Column B a) A liquid exerts pressure i) law of inertia b) Pressure decides the direction ii) is the unit of area c) Density of the substance iii) to reduce friction d) Newton’s first law of motion iv) is less than water e) Lubricant is used v) of a liquid f) Fluids vi) exert pressure g) Square meter vii) in all directions h) Density of kerosene oil viii) mass of unit volume of any substamce

a) A liquid exerts pressure in all directions.
b) Pressure decides the direction of a liquid.
c) Density of the substance mass of unit volume of any substamce.
d) Newton’s first law of motion law of inertia.
e) Lubricant is used to reduce friction.
f) Fluids exert pressure .
g) Square meter is the unit of area.
h) Density of kerosene oil is less than water.

Very Short Questions & Answers : (1 mark for each question)

Question 1.
What is the SI unit of force?
Newton.

Question 2.
What is the unit of pressure?
Pascal.

Question 3.
What is the rate of change of velocity called?
Acceleration

Question 4.
What is unit of acceleration is CGS system?
cm sec2

Question 5.
What is the CGS unit of pressure?
dyne / cm2.

Question 6.
What is density?
Mass of unit volume of any substance is known as density of the substance.

Question 7.
What is the SI unit of density?
kg m3

Question 8.
Gallon is the unit of which substance?
A gallon is a unit of measurement of density of liquids.

Question 9.
What is pressure?
The force per unit area applied on the surface of an object is known as pressure.

Question 10.
If same force is exerted on a substance and area increases then will the pressure increase or decrease?
pressure will decrease.

Question 11.
Name the instrument which is used to measure liquid pressure?
Monometer.

Question 12.
Is friction is same for sll the surfaces?
No.

Question 13.
What is the weight of substance?
The weight of a body is the force with which the earth attracts it.

Question 14.
Due to which acceleration a substance fall downwards?
Acceleration due to gravity.

Question 15.
What is the SI unit of acceleration due to gravity?
9.81 in 7 sec2.

Question 16.
Force of friction is greater in case of rough surface or smooth surface?
Rough surface.

Question 17.
Give an example to show friction produces heat.
Rubbing of our palm produces heat.

Question 18.
Can we reduce friction to zero by using lubricants?
No it is not possible to entirely eliminate friction.

Question 19.
Name the device used for neasuring force acting on an object.
Spring balance.

Question 20.
Does acceleration due to gravity depend on mass?
No.

Question 21.
When upthrust would be greater saline water or pure water?
Saline water.

Question 22.
From, which Newton’s law we get a concept about action and recation?
Newton’s thisd law of motion.

Question 23.
What ts the realtion between force, mass and accleration acting on an object?
Force = mass of the object × acceleration.

Question 24.
What is the density of water in SI sytem?
1000 kg cc.

Question 25.
Whose unit is square meter?
Square meter is the unit of area.

Question 26.
What is acceleration?
Acceleration is the rate of change of velocity with time.

Question 27.
What is SI unit of acceleration?
SI unit of acceleration is m sec2.

Question 28.
Why vehicles slow down when breaks are applied?
Because of friction.

Question 29.
What is spring balance?
Spring balance is a weighing device which is used to measuring on an object.

Question 30.
What happens when the forces ars applied on an object in the same direction?
Forces applied on an object in the same direction add to one another.

Short Questions & Answers : (2 marks for each question)

Question 1.
What is force?
A force is that physical cause which change (or tends to change) either size or shape or the state of rest or motion of the body.

Question 2.
Define friction?
Friction is a force that resists the motion of an object that is in contact with another object or material.

Question 3.
What is frictional force?
A force is that pliysical cause which clianges (or tends to change) eitrher size or shape or the state of rest or motion of the body, along the surface in contact. This is called the frictional force or the force or the force of frictions

Question 4.
Define static friction.
Static friction is the friction between two or more solid objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a slopped surface:

Question 5.
Define sliding friction.
When the object starts to move on being pulled, the frictional force which operates is known as sliding friction.

Question 6.
Write the formula of pressure per unit area in liquid.
Pressure = depth × density of liquid × acceleration due to gravity.
∴ Pressure (p) = h d g
Where h = depth, d = density, g = gravity.

Question 7.
Same volume of water is taken in a tumbler and in a pipe-shaped vessel, which will exert more pressure?
Pipe-shaped vessel will exert more pressure, because = depth × density of .liquid × acceicration due to gravity. Depth of pipe-shaped vessel is greater-than tumbler so it exerts more pressure than tumbler.

Question 8.
Why kerosene oil floats on water?
Density of kersosene oil is less than water so kerosene oil floats on water. That means, mass of 1 cc of kerosene is less than same amount of water. So kerosene oil floats on water.

Question 9.
What is acceleration to gravity?
When a body falls freely under gravity, the acceleration produced in the body due to the earth’s gravitational attraction is called the acceleration due to gravity.

Question 10.
State Pascal’s law of fluid.
Pascal’s law states that if pressure is applied at any point on an enclosed liquid, it is transmitted equally in all direction, with undiminished force, and acts right angles to the surface of the vessel.

Question 11.
Can Pascal’s law be applied on water of a pond?
No, Pascal’s law is not applicable on water of a pond as it not enclosed. Pascal’s law is applicable on enclosed liquid.

Question 12.
What is stądard atmosphric pressure.
At 45° latitude the pressure exerted by a vertical column of height 76 cm is called standard atmospheric pressure.

Question 13.
With what gravitational force earth pulls towards its gravity, a 7 kg mass object?
Gravitational pull of 7 kg mass object = 7 × 9.8 newton = 68.6 newton.

Question 14.
Will the height of mercury column be 76 cm everywhere on earth-explain.
Atmosphheric pressure is not the same at all places. Its value is maximum at sea level and decreases as we go to higher altitudes. So the height of mercury column will not be 76 cm every where on earth.

Question 15.
Give two examples of the property of equilibrium.
Two examples are-
i) Artesian well ii) water suppling system of city.

Question 16.
Define Buoyancy.
When an object is immersed in a liquid, then the liquid exerts an upward force on the object. This force is known as buoyancy.

Question 17.
Give some example to support the statement that ‘friction is important for most of our day to day activities’.
For writing with a pen or pencil, for fixing nail in the wall, for lightening of a match stick etc friction is used.

Question 18.
Why sole of shoes are grooved?
Sole of shoes are grooved to increase friction so as to provide shoes a better grip on the floor.

Question 19.
Mention Newton’s first law of motion.
A body at rest or uniform motion will continue to be at rest or uniform motion until and unless a net external force acts on it.’

Question 20.
‘Mention Newton’s second law of motion.
The greater force applied on an object, the greater is the acceleration or change of velocity of the object per second.

Question 20.
Mention Newton’s third law of motion.
When an object exerts a force on another object, the latter also exerts an equal and opposite force on the former.

Long Questions & Answers : (3 marks for each question)

Question 1.
Describe Archimede’s principle.
Archimede’s principle states that if a body is partly or wholly immersed in a liquid it experiences an apparent loss in weight due to upthrust and the apparent loss is equal to the weight of the liquid displaced by the immersed part of the body.

Question 2.
How will you proof that when an object is immersed in water the object had an apparent loss of weight?
A heavy object is hung from a spring balance. The weight of the object can be found from the recording of the spring balance. Now the object is immersed in a liquid while it hung from the spring balance. We will observe that the recording of the spring balance is lowered due to immersion of the object into the liquid. This occurs as the liquid exerts an upward force on the object.

Question 3.
What are factors that affect the upthrust?
The upthrust due to a liquid on a body depends on the following two factors :
i) the volume of the body submerged in the liquid, and
ii) the density of the liquid, in which the body is submerged.

Question 4.
Why the walls of dams at the base are made thicker?
We know that a liquid exerts the same pressure in all directions at a given depth. Pressure exerted by a liquid = height of liquid column × density of the liquid × acceleration due to gravity. So lateral pressure increases with the depth of liquid. On the upper portion of the dam density is less so the pressure is also less. Walls of dams at the base are made thicker in order to withstand the grater lateral pressure of water at that depth.

Question 5.
Why a piece of the thermocol flaots on water while an iron nail sinks in water?
The piece of thermocol floats, while the iron nail sinks. The difference in behaviour of the two obviously lies in the difference in their densities. The density of thermocol is less than the

Question 6.
Define the teums thrust and pressure. What is the relation between them?
The force acting normally on a surface is called the thrust. The thrust on unit area of the surface is called the pressure. The relation between thrust and pressure is Thrust = Pressure × Area.

Question 7.
Why ice floats on water?
The density of ice is less than the density of water. The density of ice is 0.9173 gm and that of water is 1 g cm3 .12 cc ice melts and we get 11 cc water. When ice is submerged in water it displaces water lesser than density of water. So ice floats on water.

Question 8.
Why a piece of iron unimerges in water but floats on mercury?
Density of iron is greater than water. So the weight of iron is greater than the buoyant force of water so iron sinks is water. Mercury has greater density than iron. So when iron is immersed in mercury weight of iron is less than the buoyant force of mercury so it floats on mercury.

Question 9.
It is easier for a man to swim in sea water than in fresh (or river) water, why?
The reason is that due to the presence of minerals (salt etc), the density of sea water ( = 1.026 g cm3) is more than the density of fresh (or river) water ( = 1.0 g cm3). When a small portion of the body of a mark submarges in sea water, the weight of sea water displaced becomes equal to the total weight of the body. While to displace the same weight of freșh (or river)-water, a large portion of the body will have to be submerged in water. So it becomes difficult to swim in river water.

Question 10.
An iron nail sinks in while a ship-floats, why?
If we place an iron nail on the surface of water, it sinks. This is because the density of iron is greater than of water, so the weight of the nail is more than the upthrust of water on it (or the weight of water displaced by the nail). On the other hand, ships are also made of iron, but they do not sink because the ship is hollow and the empty space in it contains_air which makes the average density of ship less than that of water. Therefore the weight of watêr displaced by the submerged part of the ship becomes equal to the total wieght of the ship and therefore the ship floats.

Question 11.
Why deep sea divers need special protective suit?
The sea divers need special protective suit to wear because in a deep sea, the total pressure exerted on the diver’s body becomes much more than his blood pressure. To withstand it, he needs to wear a special protective suit. otherwise his body would get crushed.

Question 12.
Why there is possibility of sinking of a ship when it sails from sea water to river water?
The density of sea water is more than the density of river water. According to the law of floatation, to balance the wieght of ship, a greater volume of water is required to be displaced. Therefore, when a ship sails from the sea of water of higher density to the river of water of lower density, possibility arises of sinking of ship.

Question 13.
Explian the property of equlilibrium of liquid.
If water is poured in different vessels and if they are connected with each other, it will be each other, it will be observed that the level of water in each vessel will be same. This property of liquid seeking its own level is called the property of equilibrium.

Question 14.
What is Torricellian vaccum? What is there in this vacuum?
Al m long tube is filled completely with pure and clear mercury and inverted into a through containing mercury. Some mercury from the tube flows down into the through. The mercury column now stands at a height of 76 cm above the surface of the mercury surface in the through, creating an empty space, near the closed end of the tube, this is called Torricellian vacuum. Mercury vapour fills this Torricellian vaccum.

Question 15.
What are the different types of friction?
Friction are of two types-(i) Static Friction and (ii) Sliding friction.
i) Static Friction : When a object remains static on a surface, even through it has been pulled with a force, then the frictional force which operates is known as Static Friction.
ii) Sliding Friction : When the object starts to move on being pulled, the frictional force which operates is known as Sliding Friction.

Mathematical Problems ; (3 marks for each question)

Question 1.
Mass of an object is 5 gram and pressure of 10 dyne is exerted upon it. What amount of acclaration will be created?
Force = mass of the object × acceleration
Acceleration = $$\frac{mass} {force} = \frac{10}{5}$$ cm sec2 = 2 cm sec2.

Question 2.
Mass of an object is 2 kg. What would be its weight?
Weight = Mass × Acceleration due to gravity
Acceleration due to gravity = 9.8 m sec2
∴ Weight = 2 × 9.8 newton = 19.6 newton.

Question 3.
Find out the amount of pressure at the base of a 10 m deep clear water lake. Atmospher’e pressre = 76 cm presure of mercury column and density of mercury = 13.6 gm cm3
Pressure of base of lake = Atmospheric pressure + pressure of 10 m death water. = 76 × 13.6 × 980 + 10 × 100 × 1 × 980
= 980(76 × 13.6+10000) dyne cm2
= 980 × 2033.6 dyne cm2 = 1.993 × 106 dyne cm2

Question 4.
If density of mercury is 13.6 gm cm3, then find the mass of 1 lit mercury?
Density of mercury is 13.6 gm cm3
∴ Mass of 1 cm3 mercury = 13.6 gm
Now I lit = 1000 cm3
∴ Mass of 1000 cm3 or 1 lit mercury is
1000 × 13.6 gm = 13600 gm = 13.6 kg

Question 5.
Find out the pressure of 10 cm deep water column.
pressure of water column (p) = h d g
when h = depth = 10 cm
d = density of water = 1 g cm
g = 980 cm sec2
∴ p = 10 × 2 × 980 dyne = 9800 dyne cm2

Question 6.
Find the value of standard and atmospheric pressure or 76 cm. mercury column in SI unit.
We know that pressure (p) = h d g here h = 76 cm
d = density of mercury = 13.6 g cm
g = acceleration due to gravity = 980 cm sec2
∴ p = 76 × 13.6 × 980 dyne cm2 = 1012928 dyne cm2
Newton / m2 = 10129280 Newton / m2 = 1.012928 × 107 pascal.

Question 7.
If water is kept instead of mercury in barometer then find out the height of water.
Of mercury barometer, height h1 and density d1
then p = h1 d1 g (g = acceleration due to gravity)
Again of water baremeter height is h2 and density is d2
then p = H2 d2 g
∴ h1 d1 g = h2 d2 g
Now height of mercury barometer is (h1) = 76 cm
density of mercury = 13.6 g/cc
again density of water = 1 g/cc
76 × 13.6 × g = h2 × 1 × g
∴ h2 = 76 × 13.6 = 1033.6 cm = 10.34 m

Question 8.
A square plate of side 10 m is place horizontally 1 m bleow the surface of water. The atmospheric pressure 1.013 × 105 N m2. Calculate the total thrust on the plate.
(Density of water d = 103 kg m3 g = 9.8 m s2 )