NCERT Solutions for Chapter 12 Friction Class 8 Science
Book Solutions1
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.
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 less than the static friction.
Exercises
Page Number 155
2
Four children were asked to arrange forces due to rolling, static and sliding frictions in adecreasing 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
Answer
► (c) static, sliding, rolling
Exercises
Page Number 156
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 orderwill 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
Answer
► (a) wet marble floor, dry marble floor, newspaper and towel.
Exercises
Page Number 156
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.
Answer
Frictional force is acting opposite to the movement of the book i.e. upwards.
Exercises
Page Number 156
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?
Answer
It would make it more difficult for us to walk on a soapy floor because layer of soap makes floor smooth. The coating of soapy water reduces the friction and the foot can not make a proper grip on the floor and it starts getting to slip on the floor.
Exercises
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6
Explain why sportsmen use shoes with spikes.
Answer
Sportsman use shoes with spike to increase the friction so that the shoes do not slip while they run or play.
Exercises
Page Number 156
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?
Answer
Seema will have to apply larger force since she will experience more frictional force because heavy object will be pressed hard against the opposite surface and produces more friction.
Exercises
Page Number 156
8
Explain why sliding friction is less than static friction.
Answer
Sliding friction is always less than static friction because two sliding objects find less time to get interlocked against each other’s irregularities of surfaces as a result of which they experience less friction.
Exercises
Page Number 156
9
Give examples to show that friction is both a friend and a foe.
Answer
Friction as friend:
→ We are able to walk because of friction.
→ Friction between the tip of the pen and a paper allows us to write.
Friction as foe:
→ Tyres and soles of shoes wear out because of friction.
→ Friction between the different parts of machines produces heat. This can damage the machines.
Exercises
Page Number 156
10
Explain why objects moving in fluids must have special shapes.
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.
Exercises
Page Number 156