Class 8 Friction

 

1. Define friction.

Ans. Friction is the force that opposes the relative motion between two surfaces in contact.

2. What is force of friction?

Ans. The force of friction is the resistive force acting between two surfaces in contact, which opposes motion.

3. What is the cause of friction? How can it be minimized?

Ans. Friction is caused by the roughness of surfaces and interlocking of irregularities at a microscopic level. 

It can be minimized by lubrication, polishing, using ball bearings, or streamlining objects.

4. Friction is required for walking. Explain.

Ans. Friction provides the necessary grip between our feet and the ground, allowing us to push against it and move forward. Without friction, walking would be difficult.

5. What is rolling friction?

Ans. Rolling friction is the frictional force that resists the motion of an object rolling over a surface. It is much smaller than sliding friction.

6. What is sliding friction?

Ans. Sliding friction is the resistance encountered when one surface slides over another. It is greater than rolling friction.

7. What do you mean by the term drag?

Ans. Drag is the resistive force experienced by an object moving through a fluid (air or water).

8. What is the direction of frictional force?

Ans. Frictional force always acts in the opposite direction to the motion of the object.

9. Give one difference between frictional force and gravitational force.

Ans. Frictional force opposes motion and acts only when objects are in contact, while gravitational force is an attractive force that acts between two masses at a distance.

10. Frictional force is used to stop a moving bicycle. How?

Ans. When brakes are applied, friction between the brake pads and the wheel reduces the wheel’s motion, eventually stopping the bicycle.

11. State the factors on which friction depends.

Ans. Friction depends on the roughness of surfaces in contact and the normal force pressing them together.

12. Why does a ball rolling along the ground stop after some time?

Ans. The ball stops due to rolling friction and air resistance acting against its motion.

13. Why do kabaddi players rub their hands with soil while playing?

Ans. Rubbing hands with soil increases friction, improving grip and reducing slipperiness due to sweat.

14. Give reasons:

(a) Sparks are produced when a pair of scissors is sharpened against a grinding wheel. (b) A piece of chalk wears out as it is used on a blackboard.
Ans. (a) The friction between the scissors and the wheel generates heat and tiny particles that burn as sparks.
(b) Friction between the chalk and the board causes tiny chalk particles to wear off, forming marks on the board.

15. Why are wet surfaces more slippery?

Ans. Water reduces friction by forming a thin layer between surfaces, preventing direct contact and making surfaces slippery.

16. How is the force of friction on a body in a fluid minimized?

Ans. Friction in fluids is minimized by streamlining the shape of objects, reducing surface roughness, and using smooth materials.

17. Why does a hovercraft travel much faster than a steamer pushing through water?

Ans. A hovercraft floats on a cushion of air, reducing contact with water and minimizing friction, while a steamer experiences high water resistance.

18. State the factors on which drag depends.

Ans. Drag depends on the speed of the object, the shape of the object, and the density of the fluid.

19. Give reasons:

(a) Powder is applied to a carrom board.

(b) A man walking on a street slips on a banana skin.

(c) Oil is applied to the moving part of a machine.
Ans. (a)Powder reduces friction, making it easier for the striker and coins to glide.
(b)A banana skin reduces friction, making the surface slippery.
(c)Oil reduces friction, preventing wear and tear and ensuring smooth movement.

20. Friction produces heat. Give one advantage and one disadvantage of this property. 

Ans. Advantage: Helps in lighting a matchstick.
Disadvantage: Causes wear and tear in machine parts.

21. Mention three ways in which friction between two surfaces can be minimized.

Ans. By using lubricants, ball bearings, and polishing surfaces.

22. Mention three examples by which friction between two surfaces can be increased.

Ans. By using rough surfaces, adding treads to shoes, and increasing normal force.

23. State and explain different types of friction. 

Static friction, Sliding friction, Rolling friction, Fluid friction.

Ans. Static friction: Friction that prevents an object from starting to move.
Sliding friction: Friction that resists motion when an object slides over another.
Rolling friction: Friction that occurs when an object rolls over a surface.
Fluid friction: Resistance experienced by objects moving in fluids.

24. Define: 

(a) Fluid friction (b) Streamlining

Ans. (a) Fluid friction: The resistance experienced by an object moving through a fluid (liquid or gas).
(b) Streamlining: The process of designing objects with smooth, curved shapes to reduce fluid friction.

25. What are the advantages of frictional force in our daily life?

Ans. Enables walking, writing, holding objects, driving, and stopping vehicles.

26. Define force of friction. How is the force of friction disadvantageous?Ans. 

The force of friction is the resistive force between two surfaces in contact.
Disadvantages: Causes wear and tear, produces unwanted heat, and reduces efficiency in machines.

27. Mention three disadvantages of friction between the parts of a machine.

Ans. 1, Causes wear and tear – Friction damages machine parts over time, reducing their lifespan.
2. Generates heat – The heat produced by friction can overheat and damage components.
3, Wastes energy – More force is required to overcome friction, reducing machine efficiency

27. Why does a cycle slow down when we apply brakes?

Ans. Friction between brake pads and the wheel reduces motion, slowing down the cycle.

28. Define static and sliding friction. 

Ans. Static friction: The friction that prevents motion when a force is applied.
Sliding friction: The friction acting when an object is already in motion.

29. Why is it easier to move a box that is already in motion than to get it in motion?

Ans. When a box is at rest, static friction acts between the box and the surface. Static friction is stronger because it resists the initial movement by interlocking surface irregularities.
Once the box starts moving, static friction is overcome, and sliding friction takes over. Sliding friction is weaker than static friction, so less force is needed to keep the box in motion.

30. You spill some soapy water on the floor. Would it make it easier or more difficult to walk? Explain.

Ans. It would be more difficult because soap reduces friction, making the floor slippery.

31. Is rolling friction smaller or greater than sliding friction?

Ans. Rolling friction is smaller than sliding friction.

32. Why do sportsmen use shoes with spikes?

Ans. Spikes increase friction, providing a better grip on the ground.

33. Why is it harder to drag a mat when a person is sitting on it?

Ans. The added weight increases friction, making it harder to pull.

34. Define lubricant. Give some examples.

Ans. A lubricant is a substance that reduces friction between surfaces. Examples: Oil, grease, and wax.

35. Can friction be reduced to zero by polishing or excessive use of lubricants?

Ans. No, friction cannot be completely eliminated.

36. State the factors on which fluid friction depends.

Ans. Shape, speed, and viscosity of the fluid.

37. Why do kabaddi players rub their hands with soil?

Ans. To increase friction and improve grip.

38. Why must objects moving in fluids have special shapes?

Ans. To reduce fluid friction and allow smooth movement.

39. Fill in the blanks:

(a) Friction opposes the ----- between the surfaces in contact with each other.
(b) Friction occurs due to the -------between the two surfaces.
(c) Sliding friction is ----- than the static friction.

Ans.  (a) Friction opposes the motion between the surfaces in contact with each other.
(b) Friction occurs due to the roughness between the two surfaces.
(c) Sliding friction is less than the static friction.