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.
Solution 1: (d) irrespective of whether the objects are solid, liquid or gaseous.
Friction is a force that opposes the motion of solids, liquids, and gases.
Question 2: In Figure below, 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 (↑)
Solution 2: (b) from left to right (→)
Friction often behaves in the reverse direction of motion, so the friction force will be applied from left to right.
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
(b) plastic block
(c) wooden block
(d) glass block
Solution 3: (a) stone
While rubbing the blades against a stone, the frictional force is greater, sharpening the blades.
Question 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
Solution 4: (b) polished marble surface
Because a polished marble floor has the least frictional force and inertia, a car will travel further on it.
Question 5: If we apply oil on door hinges, the friction will
(a) increases
(b) decreases
(c) disappear altogether
(d) will remain unchanged
Solution 5: (b) decreases
Oil reduces friction by acting as a lubricant.
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.
Solution 6: (d) Friction does not act on a ball moving through air.
Fluid friction acts on a body moving through the air.
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
Solution 7: (c) sprinkle talcum powder on the wooden surface.
The frictional force will be reduced by using talcum powder, allowing the ball to travel a greater distance.
Question 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.
Solution 8: (d) gravel.
Because gravel is the only path with enough frictional force to allow easy walking, the answer is (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?
Solution 9: To move the heavier block, more force will be required.
Question 10: Will force of friction come into play when a raindrop rolls down a glass window pane?
Solution 10:
When a raindrop rolls down a glass window pane, force of friction is at work.
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 spilt over it?
Solution 11: Skidding is more likely on a bicycle with worn-out tyres.
Question 12: Given figure 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?
Solution 12: As the net force on the box is zero, the friction force will be zero.
Question 13: Imagine that an object is falling through a long straight glass tube held vertical; the 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?
Solution 13: No, the object will not be subjected to any frictional forces.
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.
Solution 14: The soles of slippers become smooth after a long period of use. The friction between the sole and the floor is reduced as a result. As a result, 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?
Solution 15: Yes, there is a rolling force between a moving train's wheels and iron rails. Friction would be reduced if an air cushion is placed between the wheel and the rails.
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?
Solution 16: This cartilage deteriorates as people grow older. This increases friction, which makes movement more difficult and causes joint pain.
Question 17: While playing tug of war as shown in figure, Preeti felt that the rope was slipping through her hands. Suggest a way out for her to prevent this.
Solution 17: While playing tug of war, Preeti felt that the rope was slipping through her hands. To prevent this she could rub the soil between her hand and the rope to enhance friction.
Question 18: The handle of a cricket bat or a badminton racquet is usually rough. Explain the reason.
Solution 18: It should be rough to get a better grip on the bat since a rough surface reduces friction and allows for a stronger grip.
Question 19: Explain why the surface of mortar and pestle (silbatta) used for grinding is etched again after prolonged use?
Solution 19: To make it more effective for grinding again, increase friction.
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 the reason for your answer.
Solution 20: A marble travels the shortest distance on a sand-covered floor because it provides the most friction against its motion.
Question 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.
Solution 21: Since they had to exert pressure to set the car in movement at first, but once the car began moving, they would only have to apply force to control the car's rolling friction, which is very low.
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.
Solution 22: Friction between a knife and a spinning stone causes heat to be emitted. The amount of heat produced increases as the rotational speed increases, resulting in sparks.
Question 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.
Solution 23: As opposed to sandpaper and metal, the friction between plane paper and the metal sheet is lower. This friction works on the top layer of a metal sheet, scraping it to expose the metal underneath.
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.
Solution 24: The friction between the seat and our body is minimised if the seat cover is very smooth. As a result, when the brakes are applied in a rickshaw, we appear to fall.
Question 25: Two friends are trying to push a heavy load as shown figure below. Suggest a way which will make this task easier for them.
Solution 25: Although rolling friction is less than sliding friction, they can make their job easier by placing rollers underneath the heavy load.