Friction

1. Fill in the blanks.
(a) Friction opposes the motion between the surfaces in contact with each other.
(b) Friction depends on the nature of surfaces.
(c) Friction produces heat.
(d) Sprinkling of powder on the carrom board reduces friction.
(e) Sliding friction is less than the static friction.

2. Four children were asked to arrange forces due to rolling, static, and sliding frictions in 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

3. Alida runs her toy car on a 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

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.
Answer: The frictional force will act upwards, opposite to the direction of the book’s motion, which is downwards along the inclined surface of the desk.

5. You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer: It would make it more difficult to walk on the floor because the soapy water reduces the friction between your feet and the floor, making it slippery and increasing the chances of slipping.

6. Explain why sportsmen use shoes with spikes.
Answer: Sportsmen use shoes with spikes because the spikes increase friction between the shoes and the ground. This enhanced friction provides better grip and prevents slipping, which is essential for better performance in sports like running, football, and cricket.

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?
Answer: Seema will have to apply a larger force because the heavier box exerts more force on the floor, increasing the friction between the box and the floor. This increased friction requires more force to overcome and move the heavier box.

8. Explain why sliding friction is less than static friction.
Answer: Sliding friction is less than static friction because, during sliding, the irregularities of the two surfaces in contact do not have enough time to interlock completely. In contrast, static friction involves a greater degree of interlocking between the surfaces, requiring more force to overcome.

9. Give examples to show that friction is both a friend and a foe.
Answer:Friction as a friend:

• It helps us walk without slipping.
• It allows us to hold objects, such as a glass or a pen, firmly in our hands.
• It enables vehicles to stop when brakes are applied.

Friction as a foe:

• It causes wear and tear of machine parts, such as the soles of shoes or the treads of tires.
• It generates heat in machines, leading to energy loss.
• It makes it harder to move heavy objects.

10. Explain why objects moving in fluids must have special shapes.

Answer: Objects moving in fluids, like air and water, must have streamlined shapes to reduce fluid friction or drag. Streamlined shapes allow the object to cut through the fluid more efficiently, reducing resistance and minimizing energy loss. This is why vehicles, airplanes, and even the bodies of birds and fish are designed with streamlined shapes to move smoothly through fluids.

NCERT Exemplar:

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.

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 (¥)                                        (b) from left to right (¦)
(c) vertically downwards (œ)                            (d) vertically upwards (›)

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

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

5. If we apply oil on door hinges, the friction will

(a) increase    (b) decrease                 (c) disappear altogether      (d) will remain unchanged

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.

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.

8. In a large commercial complex, there are four ways to reach the main road. One of the paths has loose soil, the second is laid with polished marble, the third is laid with bricks, and the fourth has a 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.

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: The block with the larger mass will require a larger force to move it from the rest position because the frictional force is directly proportional to the normal force, which increases with mass.

10. Will the force of friction come into play when a raindrop rolls down a glass windowpane?
    Answer: Yes, the force of friction will come into play when a raindrop rolls down a glass windowpane, as friction always opposes the relative motion between two surfaces in contact.
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 that has lubricating oil spilled over it?
    Answer: The boy with old and used tyres is more likely to skid while moving through a patch of the road with lubricating oil spilled over it, as the worn-out tyres have less tread and hence provide less grip and higher chances of skidding.
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: No, if the magnitude of the forces applied by the two boys is equal and opposite, the box will not experience any net force and hence will not move. Without relative motion, friction will not come into play.
13. Imagine that an object is falling through a long straight glass tube held vertically; 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, the object will not experience any force of friction because there is no air (fluid) in the tube to create friction, and the object is not in contact with any surface that could provide friction.
14. You might have noticed that when used for a long time, slippers with rubber soles become slippery. Explain the reason.
    Answer: Over time, the rough surface of the rubber soles wears out due to friction, becoming smooth. This reduces the friction between the slippers and the ground, making the slippers 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 a force of friction between the wheels of a moving train and the iron rails. This type of friction is known as rolling friction. If an air cushion is introduced between the wheel and the rail, it would reduce the friction significantly, making the train move more smoothly.
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: With the wearing off of cartilage in the joints, friction between the bones increases, leading to pain and reduced mobility. This can make joint movements more difficult and may cause conditions like arthritis.
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: Preeti can rub some soil or chalk powder on her hands to increase friction between her hands and the rope, which will help prevent 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 usually rough to increase friction between the handle and the player’s hands. This prevents the bat or racquet from slipping during play, ensuring a better grip.
19. Explain why the surface of mortar and pestle (silbatta) used for grinding is etched again after prolonged use?
    Answer: The surface of a mortar and pestle (silbatta) is etched again after prolonged use to restore its roughness. Over time, the grinding process smoothens the surface, reducing friction and making it less effective. Etching increases friction, improving its grinding ability.
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 a reason for your answer.
    Answer: The marble will move the shortest distance on the surface covered with a layer of sand. This is because sand increases friction significantly, which opposes the motion of the marble more than the other surfaces, causing it to stop sooner.
21. A father and son pushed their car to bring it to the side of the 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, the father and son had to overcome static friction, which is higher than sliding or rolling friction. Once the car started moving, they only needed to overcome rolling friction, which is much less, hence requiring a smaller push to keep the car rolling.
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 cutting edge of a knife is pressed against a fast rotating stone, friction between the knife and the stone generates a significant amount of heat. The heat causes tiny particles of metal from the knife to heat up and burn, producing sparks.
23. We have two identical metal sheets. One of them is rubbed with sandpaper and the other with ordinary paper. The one rubbed with sandpaper shines more than the other. Give reason.
    Answer: Sandpaper has a rough surface that creates more friction when rubbed against the metal sheet, removing more of the oxidized layer and other impurities. This reveals the shiny surface of the metal beneath, making it shine more than the sheet rubbed with ordinary paper, which exerts less friction and removes fewer impurities.
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: A very smooth seat cover reduces the friction between the seat and your body. When the rickshaw suddenly brakes, your body continues to move forward due to inertia, but the reduced friction with the smooth seat causes you to slip rather than stay in place.
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.
    Answer: To make the task easier, the friends can place rollers or cylindrical objects like pipes under the heavy load. This reduces the sliding friction by converting it to rolling friction, which is much lower, making it easier to move the load.