NCERT solutions for class 8 Science Chapter 12 Friction

Solution for Exercise

Q.1 Fill in the blanks.
(a) Friction opposes the ______ between the surfaces in contact with each other.
(b) Friction depends on the ______ of surfaces.
(c) Friction produces ______
(d) The sprinkling of powder on the carrom board _______ friction.
(e) Sliding friction is _______ than the static friction.


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Answer :

(a) Friction opposes the __motion__ between the surfaces in contact with each other.
(b) Friction depends on the __nature__ of surfaces.
(c) Friction produces __heat__.
(d) Sprinkling of powder on the carrom board __reduces__friction.
(e) Sliding friction is __lesser__ than the static friction

Q.2 Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling


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Answer :

(c) static, sliding, rolling.

Q.3 Alida runs her toy car on a dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be
(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper.


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Answer :

(a) wet marble floor, dry marble floor, newspaper and towel.

Q.4 Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.


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Answer :

When book slides down on the desk, a frictional force acts between the book and the surface of the desk. The direction of the friction force on the book is opposite to the direction of its motion and acts in an upward direction. It is shown in the diagram below.

Q.5 You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?


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Answer :

The layer of soap makes the floor smooth due to which the friction is reduced. This makes the floor slippery and the foot cannot make a proper grip on the floor. Therefore it is difficult to walk on a soapy floor. We may slip on the floor.

Q.6 Explain why sportsmen use shoes with spikes.


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Answer :

Sportsmen use shoes with spikes to increase the friction between shoes and the surface.

Q.7 Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?


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Answer :

A heavy object produces more friction as it is pressed hard against the opposite surface. So Seema will have to apply a larger force.

Q.8 Explain why sliding friction is less than static friction.


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Answer :

Friction comes into play when irregularities present in the surfaces of two objects in contact get interlocked with each other. In sliding, the time given for interlocking is very small. Hence, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Because of this reason, sliding friction is less than static friction.

Q.9 Give examples to show that friction is both a friend and a foe.


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Answer :

Advantages of friction:
(i) We are able to walk because of friction.
(ii) Friction between the tip of the pen and a paper allows us to write.
Disadvantages of friction:
(i) Tyres and soles of shoes wear out because of friction.
(ii) Friction between the different parts of machines produces heat. This can damage the machines.

Q.10 Explain why objects moving in fluids must have special shapes.


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Answer :

When a body moves through a fluid, it experiences an opposing force which tries to oppose its motion through the fluid. This opposing force is known as the drag force. This frictional force depends on the shape of the body. By giving objects a special shape, the force of friction acting on it can be minimised. Hence, it becomes easier for the body to move through the fluid.



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