Friction is an important chapter in class 8 science books. Friction is a kind of force that opposes the motion of objects. It has both advantages and disadvantages. Friction is necessary for writing on paper and for walking. Friction causes the wear and tear of the shoe soles and the tyre. Let us learn more about friction through these NCERT Solutions for Chapter 12 Friction
Friction NCERT Solutions for Class 8
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 1.
a) motion
b) nature
c) heat
d) reduces
e) lesser
Question 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 2:
c) Static friction > Sliding friction > Rolling friction
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Question 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 3:
a) Wet marble floor < dry marble floor < newspaper and towel
Question 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.
Ans 4: When the writing desk is tilted then the book will move downwards and the force of friction will act in the opposite direction i.e. in the upward direction.
Question 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?
Ans 5: If we spill the bucket of soapy water on marble floor then we will not be able to walk. We may slip and get injured as it will totally reduce the friction between the marble floor and foot. Friction helps us to move. So, friction is mandatory in this case.
Question 6: Explain why sportsmen use shoes with spikes
Ans 6: Sportsmen use shoes with spikes because spikes increase the force of friction with the ground and prevents tha player from slipping. Football players always wear shoes having spikes in them. It creates the necessary friction and player can run and kick well without slipping on the ground.
Question 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?
Ans 7: Seema will have to apply more force as she has to move a heavier box. The heavier box will have more weight, it will apply more downward force and a greater force will be required to overcome this force to move it easily. And secondly the box is in contact with the surface so an interlocking exist between the ground and box.
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Question 8: Explain why sliding friction is less than static friction
Ans 8: Sliding friction is less than static friction because in case of static friction the contact with the surface is strong. The time for contact between surface and object is much lesser in case of sliding friction. The interlocking increases the force of friction in static friction.
Question 9: Give examples that friction is both friend and foe
Ans 9: Friction is both friend and foe. This can be understood by the following examples:
Friction Acting as Foe
Tyres of the vehicles get wear and tear due to the friction. After a particular period of time, the mechanic advises us to get the tyres changed. This increases our expenditure.
Friction produces heat when the machine runs. This causes the wear and tear of the machines.
Our shoe sole wears out in a few months or years due to friction.
Friction acting as a friend
Friction protect us from slipping when we run. The roads are made with keeping in mind the force of friction
Force of friction only allows us to walk.
Writing on paper is possible just because of friction. Try to write on a plastic sheet/polythene or slippery surface you will not be able to write properly.
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Question 10: Explain why objects moving in fluids must have special shapes.
Ans 10: Yes, the objects moving in fluids must be having a special shape because it helps them to overcome the opposing force of friction in the water. The force is known as the drag force. That’s the reason that fishes have a streamlined body and have a different shape as compared to terrestrial animals. If the objects have different shape they will be able to easily move through the water.
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