Chapter 12: Friction

Science

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  Chapter 12: Friction

  Class: VIII

  Exemplar Solutions

MULTIPLE CHOICE QUESTIONS

Question 1

Whenever the surfaces in contact tend to move or move with respect to each other, the force of friction comes into play

 (a) only if the objects are solid.

 (b) only if one of the two objects is liquid.

(c) only if one of the two objects is gaseous.

 (d) irrespective of whether the objects are solid, liquid or gaseous.

Answer 1 (d)

Question 2

 

In Fig.12.1, a boy is shown pushing the box from right to left. The force of friction will act on the box

(a) from right to left  (c) vertically downwards

(b) from left to right  (d) vertically upwards  

Answer 2 (b)

 

Question 3

 To sharpen the blade of a knife by rubbing it against a surface, which of the following will be most suitable?

(a) stone   (c) wooden block

(b) plastic block   (d) glass block

Answer 3 (a)

Question 4

A toy car released with the same initial speed will travel farthest on

 (a) muddy surface  (c) cemented surface

(b) polished marble surface   (d) brick surface

Answer 4 (c)

Question 5

 If we apply oil on door hinges, the friction will

(a) increase  (c) disappear altogether

(b) decrease   (d) will remain unchanged

Answer 5 (b)

Question 6

 Which of the following statements is incorrect?

 (a) Friction acts on a ball rolling along the ground.

(b) Friction acts on a boat moving on water.

(c) Friction acts on a bicycle moving on a smooth road.

(d) Friction does not act on a ball moving through air.

Answer 6 (d)

Question 7

 A boy rolls a rubber ball on a wooden surface. The ball travels a short distance before coming to rest. To make the same ball travel longer distance before coming to rest, he may

 (a) spread a carpet on the wooden surface.

(b) cover the ball with a piece of cloth.

(c) sprinkle talcum powder on the wooden surface.

 (d) sprinkle sand on the wooden surface.

Answer 7 (c)

Question  8

 In a large commercial complex there are four ways to reach the main road. One of the path has loose soil, the second is laid with polished marble, the third is laid with bricks and the fourth has gravel surface. It is raining heavily and Paheli wishes to reach the main road. The path on which she is least likely to slip is

(a) loose soil.  (c) bricks.

(b) polished marble.   (d) gravel.

Answer 8 (d)

 

VERY SHORT ANSWER QUESTIONS

Question 9

 Two blocks of iron of different masses are kept on a cemented floor as shown in Fig.12.2. Which one of them would require a larger force to move it from the rest position?

Answer 9

Larger force will be required to move the heavier block

Question 10

 Will force of friction come into play when a rain drop rolls down a glass window pane?

Answer 10

Yes

Question 11

 Two boys are riding their bicycles on the same concrete road. One has new tyres on his bicycle while the other has tyres that are old and used. Which of them is more likely to skid while moving through a patch of the road which has lubricating oil spilled over it?

Answer 11

The bicycle with worn out tyres is more likely to skid.

Question 12

 Fig.12.3 shows two boys applying force on a box. If the magnitude of the force applied by each is equal, will the box experience any force of friction?

Answer 12

Force of friction will be zero as the net force on the box is zero.

Question 13

 Imagine that an object is falling through a long straight glass tube held vertical; air has been removed completely from the tube. The object does not touch the walls of the tube. Will the object experience any force of friction?

Answer 13

No

SHORT ANSWER QUESTIONS

Question 14

You might have noticed that when used for a long time, slippers with rubber soles become slippery. Explain the reason.

Answer 14

When rubber soles are used for a long time, their surfaces become smooth. Hence, the friction between the sole and the floor decreases. Therefore, slippers become slippery.

Question 15

Is there a force of friction between the wheels of a moving train and iron rails? If yes, name the type of friction. If an air cushion can be introduced between the wheel and the rail, what effect will it have on the friction?

Answer 15

Yes, rolling friction. If an air cushion is introduced between the wheel and the rails, the friction will decrease.

Question 16

Cartilage is present in the joints of our body, which helps in their smooth movement. With advancing age, this cartilage wears off. How would this affect the movement of joints?

Answer 16

The wearing off of cartilage will increase the friction. As a result the movement of joints will become difficult which may lead to joint pains.

Question 17

While playing tug of war (Fig.12.4), Preeti felt that the rope was slipping through her hands. Suggest a way out for her to prevent this.

Answer 17

She may rub soil to increase friction between the rope and her hand.

Question 18

 The handle of a cricket bat or a badminton racquet is usually rough. Explain the reason.

Answer 18

To increase friction between handle of the bat and hands, to have a better grip.

Question 19

Explain why the surface of mortar and pestle (silbatta) used for grinding is etched again after prolonged use?

Answer 19

To increase friction to make it more effective for grinding again.

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 20

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 21

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 22

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 23

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 24

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.12.5. Suggest a way which will make this task easier for them. Fig. 12.5

Answer 25

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.