NCERT Class VIII Science Solutions: Chapter 12-Friction Part 4

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Question 20:

A marble is allowed to roll down an inclined plane from a fixed height. At the foot of the inclined plane, it moves on a horizontal surface (a) covered with silk cloth (b) covered with a layer of sand and (c) covered with a glass sheet. On which surface will the marble move the shortest distance. Give reason for your answer.

Answer:

On the surface covered with sand, it will cover the least distance because sand offers maximum friction against its motion.

Question 21:

A father and son pushed their car to bring it to the side of road as it had stalled in the middle of the road. They experienced that although they had to push with all their might initially to move the car, the push required to keep the car rolling was smaller, once the car started rolling. Explain.

Answer:

Because initially they had to apply force to set the card in motion but once the car started rolling, they had to apply force only to balance rolling friction of the car, the value of which is very less.

LONG ANSWER QUESTIONS

Question 22:

When the cutting edge of a knife is put against a fast rotating stone to sharpen it, sparks are seen to fly. Explain the reason.

Answer:

Friction between grinding stone and the cutting edge of the knife produces heat. As the friction is very large in this case, a large amount of heat is produced and we see sparks flying.

Question 23:

We have two identical metal sheets. One of them is rubbed with sand paper and the other with ordinary paper. The one rubbed with sand paper shines more than the other. Give reason.

Answer:

The friction between sand paper and metal sheet is very large, compared to that between the ordinary paper and the metal sheet. Thus the sand paper is able to remove the outer dull layer from the metal sheet more effectively and makes it more shining.

Question 24:

While travelling on a rickshaw, you might have experienced that if the seat cover is very smooth, you tend to slip when brakes are applied suddenly. Explain.

Answer:

If the seat cover is very smooth then the friction between our body and the seat is very small. Therefore, when the brakes are applied we tend to slip.

Question 25:

Two friends are trying to push a heavy load as shown in Fig. Suggest a way which will make this task easier for them.

As showing in fig is a Two friends are trying to push a heavy load

Two Friends Are Trying to Push a Heavy Load

As showing in fig is a Two friends are trying to push a heavy load

Answer:

They can put rollers below the heavy load. Since, the rolling friction is smaller than the sliding friction putting rollers below the heavy load will make the task easier for them.