Wed. Jul 6th, 2022

3 Mark Questions for NCERT Solutions for Class 8 Science Chapter 12

Question 1.
Give examples to show that friction is both a friend and a foe. [NCERT]

  1. Friction is a friend in the following ways — it helps us to walk, to write on the blackboard and to apply the brakes of a car.
  2. Friction is a foe in the following ways — it causes wear and tear of machinery and shoes, it produces heat in the machinery.

Question 2.
Why does friction increase if the two surfaces are pressed ?
Since the friction is due to the interlocking of irregularities in the two surfaces which slide with respect to each other, it increases when the two surfaces are pressed harder.

Question 3.
Why it is easier to move a box which is already moving than one which is at rest ?
When the box starts sliding, the contact points on its surfaces, do not get enough time to lock, into the contact points on the floor. So, the sliding friction is slightly less than static friction.

Question 4.
Why does a car stop when brakes are applied ?
In cars, the friction is increased by using brake pads in the brake system. The brake pads do not touch the wheels but when we press the brake level, these pads arrest the motion of the run due to friction and the wheels stop moving.

Question 5.
Give three factors on which the frictional force in fluids depends.
The frictional force in fluids depends on :

  • The speed of object with respect to the fluid
  • The shape of the object.
  • Nature of the fluid.

Question 6.
A labourer has to push a heavy trunk. What should be done to reduce his effort in pushing it and why ?
The labourer should put some cylindrical logs of wood under the trunk. In this way the area of contact between the two surfaces is reduced. Rolling friction is less than sliding friction.

Question 7.
Give three harmful effects of friction.

  • Friction generates heat, for e.g., between the parts of machinery.
  • Friction causes wear and tear of the rubbing surfaces e.g., soles of shoes wear out.
  • Friction reduces the speed of the body and increases the consumption of energy.

Question 8.
Why is friction essential ? Give three examples.

  • Friction is necessary between the soles of our feet and ground so that we can walk without slipping.
  • Friction helps us in writing on paper.
  • Friction makes it possible to light a match stick, by rubbing it along the rough surface of the match box.

Question 9.

  1. How do trees help reduce friction ?
  2. Why do meteors bum on entering the atmosphere ?


  1. Trees slow down strong winds and save us from wind storms.
  2. The speed of the meteor is very high. Therefore, there is a large amount of friction between the air and meteor. This generates a lot of heat and the meteor starts burning.

Question 10.
Gaurav went to the market on his bicycle. He found that he had to apply a lot of force while pedalling. Why ? What can he do ?
Gaurav had to apply more force because there was more friction between the parts of the bicycle. He should apply oil to the parts to reduce the friction.

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