CBSE Class 8 Science Chapter 9 Friction Short Answer Questions
Question 1. Imagine that an object is falling through a long, straight glass tube held vertically, and 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 frictional force because to experience the force of friction, two surfaces must be there, and there is only one surface in this case.
Question 2. While travelling on a rickshaw, you might have experienced that if the seat cover is very smooth, you tend to slip when the brakes are applied suddenly. Explain.
Answer: If the seat cover of the rickshaw is very smooth, then the friction between our body and he seat is very small. Therefore, when the brakes are applied, we tend to slip.
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Question 3. Two boys were riding their bicycles on the same concrete road. Ono has new tires on his bicycle, while the other has tires that are old and used tires. Which of them is more likely to slip while moving through a patch of the road that has lubricating oil spilled over it?
Answer: The hoy having tyres which are old and worn tires are most likely to slip because they will experience less frictional force, which is insufficient to move on the oily road.
Question 4. Explain why
1. A pencil will write on paper but not on glass.
Answer: Since, due to friction, a pencil writes on paper, but there is smoothness on the surface of glass, a pencil cannot write on glass.
2. Climbing a greasy pole is very difficult.
Answer: If a pole is polished with grease, then the surface of pole becomes smoother thus reducing friction. So, climbing a greasy pole becomes tough.
Question 5. The handle of a cricket bat or a badminton racket is usually rough. Explain the reason.
Answer: The handle of a cricket bat or badminton racquet is rough, so that while playing, the bat or badminton racquet does not slip away from the hands of the player. Roughness is responsible for the frictional force, without which gripping is not possible.
Question 6. Is there a force of friction between the wheels of a moving train and the 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 always a force of friction between the wheels of a moving train and the iron rails. The name of this friction is rolling friction, since the wheels are rolling on the track. On introducing an air cushion, the frictional force becomes less, since there is no contact between the rails and wheels.
Question 7. A father and son pushed their car 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: When the car is at rest, we have to apply a greater force because the value of static friction is greater. As the car starts moving, the friction changes into rolling friction, which is always less than the previous one. So, we have to exert a lesser force to keep it in motion.
Question 8. 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: Due to the friction between the knife and the stone, the temperature of the knife and hence the stone increases, and it increases to such a level that sparks are produced, which can be seen while sharpening it.
Question 9. 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 present in the joints ofour body reduces friction during the movement of joints. But on wearing off this cartilage, the force of friction increases, due to which the smoothness of movement decreases, and one feels joint pain.
Question 10. Explain why the surface of the mortar and pestle (silbatta) used for grinding is etched again after prolonged use.
Answer: After prolonged use, the mortar and pestle loose their roughness, due to which frictional force reduces and it does not work properly. So, we have to etch it to make it rough again.
Question 11. You might have noticed that when used for a long time, slippers with rubber soles become slippery. Explain the reason.
Answer: It is due to the continuous rubbing of the soles with the ground; the spikes on the soles get damaged slowly, and the soles become slippery.
Question 12. Explain which principle Is Involved in machine belts.
Answer: In many machines (like flour mills), belts are used to drive wheels for running the machines. Greater friction is required between the belts and machine wheels, so that the belts can drive the machine wheels properly without slipping off the wheels. So, to increase friction, machine belts are made up of special materials having rough surfaces.
Question 13. 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: Marble will move the shortest distance on the layer of sand because it will exert a greater force of friction on the marble and other two surfaces like silk cloth and glass sheet will exert a lesser friction force comparatively.
Question 14. While playing tug of war, Preeti felt that the rope was slipping through her hands. Suggest a way out for her to prevent this.
Answer: To prevent slipping of the rope from hands, Preeti has to make her hands somewhat non-smooth, so she can rub her hands by introducing sand between them.