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Friction - Class 8 : NCERT Exercise Questions


 

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.
Sol. (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 reduce 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
Sol. (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
Sol. (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.
Sol. When book starts sliding down on the writing desk, frictional force will act between the book and the surface of the desk. The direction of the frictional on the book will be opposite to the direction of motion of book. Hence, frictional force will be in upward direction as book is sliding in the downward direction.

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?
Sol. Friction makes it possible for us to walk on the floor. If a bucket of soapy water spills on the floor, then the force of friction between the ground and feet will decrease. Hence, it would become more difficult to walk on the soapy floor.

Q.6 Explain why sportsmen use shoes with spikes.
Sol. Sportsmen use shoes with spikes because spikes increase friction and give better grip with the ground. Thus, it avoids slipping of sportsmen while playing or running.

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?
Sol. Seema will have to apply a larger force because frictional force is directly proportional to the mass. Hence, heavier box will put more force on the floor as compared to the lighter box which means more force will have to be applied to overcome the more friction due to heavier box.

Q.8 Explain why the sliding friction is less than the static friction.
Sol. Friction is basically the irregularities present between two objects in contact which gets interlocked with each other. In case of sliding, there is not much time available for objects to get interlocked. Hence, less force is required to overcome this friction. Thus, sliding friction is less than static friction.

Q.9 Give examples to show that friction is both a friend and a foe.
Sol. Following are some examples that show friction is friend:
(i) We are able to write because of friction between ball point of pen and the paper surface.
(ii) We are able to walk because of friction.
(iii) We are able to write with chalk on blackboard because of friction.
(iv) We are able to drive the automobile because of friction between the tyres and the road.
(v) We are able to fix a nail in the wall because of friction.
Following are some examples that show friction is foe:
(i) Soles of shoes and tyres wear out because of friction.
(ii) Friction wears out the materials whether they are screws or ball bearings.
(iii) Steps of foot over-bridges at railway stations get worn-out because of friction.
(iv) Machines get heated up because of friction.
(v) When a tyre deflates, it is difficult to move the vehicle because of increased friction between the tyre and road surface.

Q.10 Explain why objects moving in fluids must have special shapes.
Sol. Whenever any object moves through a fluid, friction force tries to oppose its motion through the fluid. This opposing force is known as the drag force. To reduce this force, the bodies of objects needs to minimise its surface i.e. sometime spherical shapes or other streamlined shape with smooth surface. Example: Fishes have streamlined bodies to reduce the water drag.



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