Force and Pressure
Solutions for Physics, Class 8, ICSE
Exercise 3 Assertion Reason Type
3 questionsAssertion (A) : A force when applied on a body changes its mass.
Reason (R) : Force is the cause which changes the size or shape of the body.
- Both A and R are true and R is the correct explanation of A
- Both A and R are true and R is not the correct explanation of A
- Assertion is true but reason is false
- Assertion is false but reason is true
Answer:
Assertion is false but reason is true
Explanation
Assertion (A) is false because a force does not change the mass of the body on which it is applied.
Reason (R) is true because force is that cause which changes the state of a body (either the state of rest or the state of motion) or changes the size or shape of the body.
Assertion (A) : In heavy container trucks, more number of tyres are used to reduce the pressure on the ground.
Reason (R) : Larger the surface area, less is the pressure for the same thrust.
- Both A and R are true and R is the correct explanation of A
- Both A and R are true and R is not the correct explanation of A
- Assertion is true but reason is false
- Assertion is false but reason is true
Answer:
Both A and R are true and R is the correct explanation of A
Explanation
Assertion (A) is true because in heavy container trucks, more number of tyres increases the area of contact and thereby reduces the pressure on the ground and if there are only four tyres in a heavy truck, due to less area the pressure applied by the truck will be more and hence its speed will get affected.
Reason (R) is true because the pressure on a surface depends inversely on the area of the surface on which thrust acts so that smaller the surface area, more is the pressure exerted by the thrust and larger the surface area, lesser is the pressure exerted by the same thrust.
So, here reason justifies the assertion.
Assertion (A) : Camels can walk easily in deserts without their feet sinking in sand.
Reason (R) : Camels have large broad feet with a wide base, thus increasing the pressure exerted on sand.
- Both A and R are true and R is the correct explanation of A
- Both A and R are true and R is not the correct explanation of A
- Assertion is true but reason is false
- Assertion is false but reason is true
Answer:
Assertion is true but reason is false
Explanation
A camel’s broad feet spread its weight over a larger area, so the pressure it exerts on the sand is lower; this prevents its feet from sinking. Hence the Assertion is correct. The Reason, however, is incorrect because the feet decrease, not increase, the pressure on the sand.
Exercise 3 Fill In The Blanks
1 questionFill in the blanks:
(a) 1 kgf = ............... N (nearly).
(b) Moment of force = ............... x distance of force from the point of turning.
(c) In a door, handle is provided ............... from the hinges.
(d) The unit of thrust is ............... .
(e) Thrust is the ............... force acting on a surface.
(f) Pressure is the thrust acting on a surface of ............... area.
(g) The unit of pressure is ............... .
(h) Pressure is reduced if ............... increases.
(i) Pressure in a liquid ............... with the depth.
(j) The atmospheric pressure on earth surface is nearly ............... .
Answer:
(a) 1 kgf = 10 N (nearly).
(b) Moment of force = force x distance of force from the point of turning.
(c) In a door, handle is provided farthest from the hinges.
(d) The unit of thrust is Newton .
(e) Thrust is the normal force acting on a surface.
(f) Pressure is the thrust acting on a surface of unit area.
(g) The unit of pressure is Pascal.
(h) Pressure is reduced if surface area increases.
(i) Pressure in a liquid increases with the depth.
(j) The atmospheric pressure on earth surface is nearly 105 Pa.
Exercise 3 Long Answer Type Questions
17 questionsAnswer:
It is easier to turn the steering wheel of large diameter as larger the diameter, larger is the perpendicular distance from the axis of rotation so less force is applied to generate same amount of turning effect as compared to steering wheel of small diameter.
Answer:
The hand flour grinder is provided with a handle near the rim which is at a maximum distance from the axis of rotation so that it can easily be rotated at its centre by applying a small force at the handle.
Answer:
It is easier to open the door by pushing it at its free end because larger the perpendicular distance from the axis of rotation, less is the force required to open it.
Answer:
A potter's wheel is pivoted at the centre. When the potter applies a force through the stick near the rim of the wheel, the point of application of force is at a maximum distance from the axis of rotation, hence less force is required to rotate the wheel.
Answer:
(a) Wide wooden sleepers are placed below the railway tracks to increase the surface area of the tracks in contact with the ground. This reduces the pressure exerted by the rails on the ground.
(b) The foundation of a tall building is made wider because the weight of the building will act on larger surface area exerting less pressure on the ground (as pressure is inversely proportional to surface area). This avoids sinking of building into the earth.
Answer:
The following experiment demonstrates that a liquid exerts pressure at the bottom of the container in which it is kept:

- Take a glass tube and tie a balloon at its lower end.
- Hold it vertically straight as shown in the above figure.
- Pour some water in the tube.
Observations — After sometime we will notice that the balloon bulges out as shown in figure (b) above.
Conclusion — Balloon bulges out because water column exerts pressure at its bottom. The force on the balloon is equal to the weight of the water column known as thrust.
So, Pressure = = , where W is weight of the water column (thrust) and A is surface area.
Answer:
The following experiment demonstrates that a liquid exerts pressure sideways also:

- Take a glass tube closed at one end and having an opening in its side near the bottom.
- Tie a deflated balloon at the side opening of the tube.
- Hold the tube vertically straight as shown in the above figure.
- Pour some water in the tube.
Observations — After sometime we will notice that the balloon bulges out as shown in figure (b) above.
Conclusion — From the above experiment it is concluded that a liquid exerts pressure sideways also.
Answer:
The experiment is described below:

- Take a glass tube open at both ends. Hold it vertically.
- Tie a balloon at its lower end.
- Pour some water in the tube.
- Balloon starts bulging out as shown in figure (a) above.
- Add more water in the tube.
- Balloon will bulge more as shown in figure (b) above.
Conclusion — From the above experiment it is concluded that when the height of the water column increases, more pressure is exerted on the balloon and it bulges out more. This shows that the liquid pressure at a point increases with the increase in height of the liquid column above that point.
Answer:
The following experiment demonstrates that at a given depth, a liquid exerts same pressure in all directions:

- Take a balloon and fill it with water.
- Tie the mouth of the balloon.
- Make holes in the balloon by inserting pins at several places in all directions.
Water comes out through each hole which shows that liquid exerts same pressure in all directions.
Answer:
The following experiment demonstrates that liquid pressure depends on the density of liquid:

- Take two identical glass tubes open at both the ends. Mark them A and B.
- Hold both the tubes vertically and tie an inflated balloon to both the tubes at their lower ends.
- In tube A pour some water.
- In tube B pour some concentrated sugar solution such that its height is same as that of height of water in tube A.
Observation — It is noticed that the balloon attached to tube B bulges out more as compared to balloon attached to tube A as shown in the above figure.
Conclusion — Density of sugar solution is more than water so it exerts more pressure which shows that liquid pressure depends on the density of liquid.
Answer:
The following experiment demonstrates that air exerts pressure:

- Take a glass filled with water up to its brim and place a post card on top of it.
- Press the palm of your one hand on top of post card and then invert the filled glass upside down.
- Now gently remove your hand from the post card to release it.
Conclusion — It is observed that post card does not fall from the glass because the atmospheric pressure is acting upwards on the post card from outside the glass which overcomes the pressure on post card due to water in the glass.
Answer:
Crushing tin can experiment

- Take a thin walled tin can with airtight stopper. Remove the stopper.
- Fill the can partially with water. Heat the can over the flame of a burner till water boils.
- Now the air pressure inside and outside the can is same.
- When steam comes out of the opening put the stopper.
- Remove the can from the burner.
- Place the can in the tub and pour cold water on the can.
It is observed that the can collapses. The reason is that when steam comes out it takes away most of the air from the can. When cold water is poured on the can, steam condenses into water leaving a partial vacuum in the can. The air pressure outside is more than inside which exerts force on the can causing it to collapse.
From this experiment it is concluded that air exerts pressure.
Answer:
There are two forces acting on the water inside the dropper one is liquid pressure from inside and atmospheric pressure from outside. Atmospheric pressure acting from outside balances the liquid pressure from inside so water does not come out from the dropper. When bulb is pressed the liquid pressure increases than atmospheric pressure and water comes out from the dropper.
Answer:
Two holes are made in a sealed oil tin to take out oil from it because through one hole atmospheric pressure acts due to air entering through it and through another hole the oil comes out easily.
Exercise 3 Match The Columns
1 questionAnswer:
Column A | Column B |
---|---|
(a) Camel | (ii) broad feet |
(b) Truck | (iii) six or eight tyres |
(c) Knife | (iv) sharp cutting edge |
(d) High building | (i) broad and deep foundation |
(e) Thrust | (vi) N |
(f) Moment of force | (vii) N m |
(g) Atmospheric pressure | (v) atm |
Exercise 3 Numericals
13 questionsAnswer:
Given:
Force (f) = 5.0 N
Distance (d) = 10 cm = 0.1 m
Moment of force = ?
Moment of force = force (f) x distance (d)
= 5.0 x 0.1
= 0.5 N m
So, Moment of force = 0.5 N m.
Answer:
Given:
Moment of force = 2.0 N m
Diameter = 2 m
Radius = 1 m
Therefore perpendicular distance (d) = radius = 1 m
Force (f) = ?
So, minimum force required is 2 N.
Answer:
Given:
Weight = Thrust = 60 N
Area = 15 cm x 20 cm = 300 cm2 = = 0.03 m2
Pressure = ?
So, Pressure = 2000 Pa.
Answer:
Given:
Thrust = Weight = 60 kgf
1 kgf = 10 N
∴ 60 kgf = 60 x 10 = 600 N
So, Thrust = 600 N
Area = 2.5 cm x 0.5 cm = 1.25 cm2 = = 0.000125 m2
Pressure =?
So, Pressure = 48,00,000 or 4.8 x 106 Pa.
Answer:
First case :
Thrust = Weight = 2 kgf
Area = 20 cm x 10 cm = 200 cm2
Pressure = ?
So Pressure exerted by brick in first figure is 0.01 kgf cm-2.
Second case :
Thrust = Weight = 2 kgf
Area = 5 cm x 10 cm = 50 cm2
Pressure = ?
So Pressure exerted by brick in second figure is 0.04 kgf cm-2.
Third case :
Thrust = Weight = 2 kgf
Area = 20 cm x 5 cm = 100 cm2
Pressure = ?
So, Pressure exerted by brick in third figure is 0.02 kgf cm-2.
Exercise 3 Objective Type Questions
11 questionsAnswer:
Applying the force farthest from the pivoted point.
Reason — Larger the perpendicular distance of the point at which the force is applied from the pivoted point, less force is needed to obtain a given moment of force for turning a body.
Answer:
5 kgf
Reason — A body when placed on a surface exerts a thrust on the surface which is equal to its own weight. So thrust = weight of the body = 5 kgf.
Answer:
suction pads
Reason — Lizards can stay and move on walls because their feet behave like suction pads so they remain pressed against the wall due to atmospheric pressure.
Answer:
increase in depth
Reason — Pressure of liquid at a point increases with the height of the liquid column above it, so as depth increases pressure inside liquid increases.
Answer:
the atmospheric pressure decreases
Reason — At higher altitudes, atmospheric pressure decreases but our body pressure remains the same. So, inside pressure becomes more than atmospheric pressure which causes nose bleeding.
Exercise 3 Short Answer Type Questions
24 questionsAnswer:
(a) When a force is applied on a rigid body, it produces only change in motion of the body. The force does not cause any change in the inter-spacing between its constituent particles.
(b) When a force is applied on a non-rigid body, it causes both change in its size or shape and motion in it.
Answer:
(i) In figure (a), when a force F is applied on the ball, the ball starts moving in a straight line in the direction of force F and continues to move in that direction. This is called linear motion.

(ii) In figure (b), when a force F is applied on the wheel pivoted at a point, the force turns the wheel in anticlockwise direction about the axis of rotation producing turning motion.

Answer:
The expression for the moment of force about a given axis of rotation is —
Moment of force about a given axis = Force x perpendicular distance of force from the axis of rotation.
Answer:
The moment of given force about a given axis of rotation is decreased by either decreasing the force or by decreasing the perpendicular distance of force from the axis of rotation.
Answer:
If the effect on the body is to turn it clockwise, the moment of force is called clockwise moment. If the effect on the body is to turn it anti-clockwise, the moment of force is called anti-clockwise moment.
Answer:
The effect of thrust depends on the area of the surface on which it acts. Smaller the area of the surface on which a thrust acts larger is its effect and effect of thrust is less on a larger surface area.
Answer:
The bottom part of the foundation of a building is made wider so that the weight of the building will act on larger surface area exerting less pressure on the ground (as pressure is inversely proportional to surface area). This avoids sinking of buildings into the earth.
Answer:
A sharp knife has sharp edges that provides less surface area of contact as compared to a blunt knife which has more surface area of contact. So pressure applied by the force is more in sharp knife which makes it easier to cut than a blunt one.
Answer:
(i) A gum bottle has broader base and narrower neck and when it is placed upside down there will be no change in thrust as thrust is the weight of the body that is exerted on the surface.
(ii) When gum bottle is placed upside down, surface area is less so pressure will be more as pressure is inversely proportional to surface area.
Answer:
The two factors on which the pressure at a point in a liquid depends are:
- The height of the liquid column — Liquid pressure increases with the height of the liquid column above the point.
- The density of the liquid — Liquid pressure increases with the increase in the density of the liquid.
Answer:
The blood in the veins of our body exerts a pressure which is slightly more than the atmospheric pressure which makes the effect of atmospheric pressure ineffective. So we do not feel uneasy even under the enormous atmospheric pressure.
Exercise 3 Think And Answer
7 questionsAnswer:
No, the body will not rotate because
Turning effect = Force x perpendicular distance of the force from the pivoted point.
= F x 0 = 0.
Perpendicular distance is 0 or force is parallel to the point of application of force, hence the body will not rotate.
Answer:
A man exerts more pressure on the surface while walking than standing because while walking one foot is in contact with the floor so surface area is less but while standing both foot are in contact with the floor providing a large surface area. Since pressure is inversely proportional to surface area so pressure exerted while walking is more as compared to pressure exerted on standing.
Answer:
The broad feet of camels and elephants increases the surface of the feet in contact with the ground. This reduces the pressure exerted on the ground as pressure is inversely proportional to surface area.
Thus, this helps camels to walk easily on the desert sand without their feet sinking in it. The large body weight of elephants is supported by their broad feet helping them to move around comfortably.
Answer:
A sharp pin has less surface area as compared to a blunt pin, so for a given applied force sharp pin exerts more pressure than blunt pin and penetrates a surface like wood or wall easily. So, it works better than a blunt pin.
Answer:
The diagram demonstrates that liquid pressure at a point increases with the height of the liquid column above it as water flowing out from upper hole falls near the cylinder while water from lower hole falls far from the cylinder.
Answer:
The pressure at a point due to a liquid increases with the increase in height of the liquid column above it, so to withstand the increasing pressure of water a dam has broader walls at the bottom than at the top.
In the figure given below, the increasing length of arrows in water represents the increasing pressure on the wall of the dam towards the bottom.

Answer:
Sea divers need to wear specially designed swim-suit because in deep sea, the total pressure exerted on the diver's body is much more than his blood pressure. To withstand it, they need to wear a specially designed swim-suit, made from glass reinforced plastic or cast aluminium. The pressure inside the suit is maintained at one atmosphere. This allows the diver to withstand the enormous external pressure and be safe underwater.
Exercise 3 True Or False
1 questionWrite true or false for each statement:
(a) The S.I. unit of force is kgf.
(b) A force always produces both the linear and turning motions.
(c) Moment of force = force x perpendicular distance of force from the pivoted point.
(d) Less force is needed when applied at a farther distance from the pivoted point.
(e) For a given thrust, pressure is more on a surface of large area.
(f) The pressure on a surface increases with an increase in the thrust on the surface.
(g) A man exerts same pressure on the ground whether he is standing or he is lying.
(h) It is easier to hammer a blunt nail into a piece of wood than a sharply pointed nail.
(i) The S.I. unit of pressure is pascal.
(j) Water in a lake exerts pressure only at its bottom.
(k) A liquid exerts pressure in all directions.
(l) Gases exert pressure in all directions.
(m) The atmospheric pressure is nearly 105 Pa.
(n) Higher we go, greater is the air pressure.
Answer:
(a) False
Correct Statement — The S.I. unit of force is newton.
(b) False
Correct Statement — A force can produce either linear motion or turning motion.
(c) True
(d) True
(e) False
Correct Statement — Pressure is defined as thrust per unit area. Hence, for a given thrust, pressure is less on a surface of large area.
(f) True
(g) False
Correct Statement — A man exerts more pressure while standing than lying on the ground as area of contact is less in case of standing as compared to lying on the ground.
(h) False
Correct Statement — Incase of blunt nail, the thrust exerted on hammering acts on a larger area so less pressure acts on the piece of wood making it difficult to hammer the nail.
(i) True
(j) False
Correct Statement — Water in a lake exerts pressure in all directions.
(k) True
(l) True
(m) True
(n) False
Correct Statement — Higher we go, lesser is the air pressure.