Chapter 12: Fraction

MULTIPLE CHOICE QUESTIONS

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: (d)irrespective of whether the objects are solid, liquid or gaseous.
[Friction arises whenever two surfaces interact and try to move relative to each other, no matter if they are solids, liquids, or gases.]

2.

         Fig. 12.1
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 (←)    (b) from left to right (→)    (c) vertically downwards (↓)  (d) vertically upwards (↑)

Answer: (b) from left to right (→)   

[ Friction always acts in the opposite direction to the motion . Since the boy pushes the box left, friction acts toward the right .]

3.To sharpen the blade of a knife by rubbing it against a surface, which of the following will be most suitable?
(a) stone     (b) plastic block     (c) wooden block   (d) glass block

Answer:  (a) stone

[ Stone provides rough surface with high friction, essential to sharpen the blade by wearing away metal.]

4.A toy car released with the same initial speed will travel farthest on
(a) muddy surface    (b) polished marble surface  (c) cemented surface  (d) brick surface

Answer: (b) polished marble surface

[ Polished marble offers the least friction, allowing the toy car to travel farthest with the same initial speed.]

5. If we apply oil on door hinges, the friction will
(a) increase      (b) decrease  (c) disappear altogether    (d) will remain unchanged

Answer: (b) decrease

[ Oil acts as a lubricant and forms a thin layer between the surfaces, reducing direct contact and hence decreasing friction.]

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

[ This is incorrect because air exerts fluid friction (drag) on a ball moving through it.]

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: (c) sprinkle talcum powder on the wooden surface.

[ Talcum powder reduces friction by acting as a lubricant between the ball and wooden surface.]

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.       (b)  polished marble.   (c) bricks.     (d) gravel.

Answer: (d) gravel.

[ Gravel surface is rough and provides more friction even when wet, so chances of slipping are least. Polished marble is very smooth and becomes very slippery in rain, while loose soil and bricks offer less grip compared to gravel.]

VERY SHORT ANSWER QUESTIONS

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?

                Fig. 12.2

Answer: The block with 2 units of mass requires a larger force . This is because friction increases with mass ; a heavier object presses harder against the surface, creating greater resistance .

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

Answer: Yes, force of friction comes into play when a raindrop rolls down a glass pane. It acts between the water and glass, slowing down the motion.

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: The boy with old and used tyres is more likely to skid. Old tyres have smoother surfaces and provide less friction, especially on oily roads, while new tyres have better grip and reduce slipping.

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?

                  Fig. 12.3

Answer: No, the box experiences no friction . Since equal and opposite forces are applied, the net force is zero, meaning there is no tendency for the box to move .

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: No, it will not experience any friction because air is removed (no fluid friction) and no contact with walls exists.

SHORT ANSWER QUESTIONS

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

Answer: When slippers with rubber soles are used for a long time, their surface becomes smooth due to wear and tear. This reduces friction, so they lose grip and become slippery.

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: Yes, there is friction between the wheels of a moving train and the iron rails. It is rolling friction.

If an air cushion is introduced, contact between wheel and rail reduces, so friction will decrease greatly, making motion smoother.

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: Cartilage reduces friction between bones in a joint. When it wears off with age, bones may rub against each other, causing pain, stiffness and difficulty in movement. This makes joint movement less smooth and more uncomfortable.

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.

                Fig. 12.4

Answer: Preeti should rub her hands with soil . This increases the friction between her hands and the rope, providing a better grip and preventing the rope from slipping .

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

Answer: The handle of a cricket bat or a badminton racquet is made rough to increase friction between the hand and the handle. Higher friction provides a better grip, preventing the bat or racquet from slipping while playing.

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

Answer: After prolonged use, the surface of the mortar and pestle (silbatta) becomes smooth due to continuous rubbing. This reduces friction, making grinding less effective. Therefore, the surface is etched again to make it rough and increase friction for efficient grinding.

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: The marble will move the shortest distance on sand (b) .This is because sand provides maximum friction among the three surfaces, which opposes motion strongly and stops the marble quickly.

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: Initially, they must overcome static friction, which is higher. Once the car starts moving, rolling (or kinetic) friction acts, which is smaller. Hence, less force is needed to keep the car moving than to start it. 

LONG ANSWER QUESTIONS

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: When the knife edge is pressed against the fast rotating stone, strong friction is produced. This friction generates a lot of heat at the contact point. The metal becomes very hot and small particles get detached and burn in air, appearing as sparks.

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: Sand paper is rough and removes the upper dull layer of the metal sheet more effectively. This exposes the fresh, smooth metal surface underneath, which reflects light better and appears shiny. Ordinary paper is smooth and cannot remove the oxide or dirt layer properly, so it does not produce much shine.

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: When the rickshaw stops suddenly, your body tends to continue moving forward due to inertia. If the seat cover is very smooth, there is very little friction between your body and the seat, so it cannot stop your motion easily. As a result, you slip forward. A rough seat cover provides more friction and prevents slipping.

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: The friends should place rollers or wheels under the heavy load . This makes the task easier because rolling friction is much less than sliding friction. By converting sliding into rolling , the opposing force is reduced, requiring less effort to move the box .