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Answer :

When a book slides down on the desk, a frictional force acts between the lower surface of the book that is in contact with the desk and the upper surface of the desk that is in contact with the book. That means the frictional force comes into play when the minute roughness on the surface of the book gets interlocked with the minute roughness on the surface of the desk.

The direction of the frictional force on the book is opposite to the direction of its motion of the book. As the book is sliding downwards so the frictional force acts in the upward direction.

It acts as shown in the diagram below :-

- 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.
- 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
- 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.
- 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?
- Q.6 Explain why sportsmen use shoes with spikes.
- 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?
- Q.8 Explain why sliding friction is less than static friction.
- Q.9 Give examples to show that friction is both a friend and a foe.
- Q.10 Explain why objects moving in fluids must have special shapes.

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