NCERT Solutions for Class 8th Science

Chapter 12 – FRICTION

(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)

 

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) Sprinkling of powder on the carrom board ________ friction.

          (e) Sliding friction is ___________ than the static friction.

Ans   (a) Friction opposes the _relative 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 _less_ 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

Ans   (c) Static, sliding, rolling

Q.3      Alida runs her toy car on 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

Ans     (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.

Q.5      You spill a bucket of soapy water on a marble floor accidently. 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 the sliding friction is less than the static friction.

Ans     Friction is caused by the interlocking of irregularities in the two surfaces and when a body starts sliding              the contact points on its surface, do not get enough time to lock into the contact points on the floor.                      Hence, the sliding friction is slightly smaller than the static friction.

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

Ans      Friction as a friend:

            1. We are able to walk because of friction between floor and our feet.

            2. We can write with pen or pencil because of friction.

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