# Friction Class 8 Important Questions and Answers

Important questions of Class 8 Science Chapter 12 Friction is given below. These important questions will help students while preparing for the exam.  Practising these important questions will analyse their performance and work on their weak points. Score well in exam of Class 8 Science by going through these important questions. Students of Class 8 can download important questions of Class 8 Science Chapter 12 Friction PDF by clicking the link provided below.

## Important Questions of Class 8 Science Chapter 12 Friction

Here you can get Class 8 Important Questions Science based on NCERT Text book for Class 8. Science Class 8 Important Questions are very helpful to score high marks in board exams. Here we have covered Important Questions on Friction for Class 8 Science subject.

1. What is force of friction?
The force acting on a moving body which acts opposite to the direction of motion is called force of friction.

2. Is the friction the same for all the surfaces?
No, friction is not the same for all the surfaces.

3. Does friction depend on the nature of objects?
Yes, friction depends on nature of objects.

4. Does friction depend on smoothness of the surface?
Yes, friction depends on the smoothness of the surface.

5. Which surface produces more friction?
Rough surface produces more friction.

6. Which type of surface produces less friction?
Smooth surface produces less friction.

7. What is the direction of frictional force?
The direction of frictional force is opposite to the motion.

8. What is the cause of friction between two surfaces?
Friction is caused by the irregularities on the two surfaces in contact.

9. Which surface has more irregularities, smooth or rough surface?
Rough surface has more irregularities.

10. How many types of friction are there?
There are three types of friction: (i) Static friction (ii) Sliding friction (iii) Rolling friction

11. Which is less, sliding friction or static friction?
Sliding friction is smaller than the static friction.

12. Which force helps to write by chalk on the blackboard?
Frictional force.

13. Which is easier to hold, an earthen pot or a glass tumbler?
Holding earthen pot is easier than holding a glass tumbler.

14. Why is it difficult to move on a wet marble floor?
Wet marble floor has less friction, so it is difficult to move on it.

15. What does friction do to soles of our shoes?
Friction causes wear and tear in soles of our shoes.

16. Why does a matchstick catch fire when it is rubbed on a rough surface?
Matchstick catches fire due to friction.

17. What happens when there is no friction between the chalk and the blackboard?
We cannot write on the blackboard.

18. What would happen when an object starts moving if there is no friction?
The object would never stop if there is no friction.

19. What does friction produce?
Heat.

20. Write one harm of friction.
Friction wears out the materials, like machine parts.

21. Why does jar of a mixer become hot when it is run for few minutes?
The jar of a mixer becomes hot due to friction.

22. Why do the tyres of cars and trucks are treaded?
The treaded tyres of cars and trucks provide better grip with the ground.

23. Why do Kabaddi players rub their hands with soil?
Kabaddi players rub their hands with soil for a better grip of their opponents.

24. Give two examples where friction is undesirable.
(i) We sprinkle fine powder on carrom board to reduce friction.
(ii) Drops of oil are poured on the hinges of a door so that the door moves smoothly.

25. What do you mean by lubricants?
Answer: The substances which are used to reduce friction are called lubricants.

26. Why are oil, creams and grease called lubricants?
Oil, creams and grease are called lubricants because they are used to reduce friction.

27. How are the shoes made to counter the friction and make better grip?
They have grooves on their soles.

28. What is used in cycle brakes to increase friction?

29. Name the substance which is used in carrom board to reduce friction.
Talcum powder.

30. What is avoided between two surfaces to make movement smooth?
Interlocking of irregularities is avoided.

31. Can we eliminate friction completely?
No, we cannot eliminate friction completely.

32. What is used to reduce friction in machines where lubrication are not advisable?
Air cushion.

33. Why do pieces of luggage fitted with rollers?
Pieces of luggage are fitted with rollers to reduce single friction so that children can pull such pieces of luggage easily.

34. Name two methods of reducing friction.
(i) By rolling
(ii) By sliding.

35. How sliding friction is replaced by rolling in machines?
By using ball bearings.

36. Name any machine in which ball bearing is used to reduce friction.
Ceiling fan.

37. What is drag?
The frictional force exerted by fluids is called drag.

38. What are fluids?
Commonly gases and liquids are called fluids.

39. What does frictional force exerted on an object in a fluid depend on?
Friction depends on the speed with respect to the fluid and the nature of fluid.

40. Name the special shape given to objects moving in fluid.
Streamlined shape.

41. Name some objects which have streamlined body.
Boats, aeroplanes and ships.

42. When does rolling friction come into play?
When a body rolls on other body then rolling friction comes into play.

1: Why a vehicle slows down when brakes are applied?

Answer: This is because of the force of friction between the brake and the wheel of the moving vehicles.

2: Why it is difficult to move on wet floor?

Answer: It is difficult to walk on a wet floor because layer of water makes floor smooth. The coating of water reduces the friction and the foot cannot make a proper grip on the floor and it starts getting to slip on the floor.

3: Define force of friction with an example.

Answer: Friction: Friction is the force which resists the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.

4: What are the factors affecting friction?

Answer: Friction depends partly on the smoothness of the contacting surfaces, a greater force being needed to move two surfaces past one another if they are rough than if they are smooth. However, friction decreases with smoothness only to a degree; friction actually increases between two extremely smooth surfaces because of increased attractive electrostatic forces between their atoms. Friction does not depend on the amount of surface area in contact between the moving bodies or (within certain limits) on the relative speed of the bodies. It does, however, depend on the magnitude of the forces holding the bodies together.

5: Give an example to show that friction increases if the two surfaces are pressed harder.

Answer: Dragging a mat when nobody is sitting on it easier than dragging the same mat when a person is sitting on it.

6: Why sliding friction is slightly smaller than static friction?

Answer: Sliding friction is slightly smaller than static friction because two sliding objects find less time to get interlocked against each other’s irregularities of surfaces as a result of which they experience less friction.

7: What would have happened if there were no friction between chalk and board?

Answer: We could not write with chalk on board.

8: When we strike a matchstick against the rough surface, it produces fire. Why so?

Answer: Striking a matchstick produces fire by friction.

9: Give an example to support that friction leads to wastage of energy.

Answer: Friction produces heat, when a machine is operated, heat is generated which causes much wastage of energy.

10: How can we increase friction of soles of shoes?

Answer: Soles of shoes are grooved so that a better grip is provided to the surface and thus can move safely and freely on floors.

11: How can we increase friction of tyres of vehicles?

Answer: The tyres of vehicles are treaded to provide better grip with the ground.

12: Give an example where we have to minimise friction.

Answer: Powder is sprinkled on the carom board to reduce friction and increase efficiency.

13: What is the function of lubricants?

Answer: A lubricant is a substance introduced to reduce friction between moving surfaces. It performs following functions:

• Reduce friction
• Transfer heat
• Carry away contaminants & debris
• Transmit power
• Protect against wear
• Prevent corrosion
• Seal for gases
• Stop the risk of smoke and fire of objects

14: Why friction can never be entirely eliminated?

Answer: Friction can never be entirely eliminated, because no surface is perfectly smooth, there are always some irregularities present in it.

15: What is rolling friction?

Answer: Rolling friction is the force that resists the rolling of a wheel or other circular object along a surface caused by deformations in the object and or surface. Generally the rolling friction is smaller than sliding friction. example: One of the most common examples of rolling resistance is the movement of motor vehicle tires on a road.

16: Why it is convenient to pull luggage bags fitted with rollers?

Answer: This is because rolling reduces friction; it is always easier to roll than to slide a body over another.

17: What do you mean by sliding friction?

Answer: Friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground) is called sliding friction.

18: Differentiate between sliding and rolling friction.

Answer: Friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground) is called sliding friction, but Rolling friction is the force that resists the rolling of a wheel or other circular object along a surface caused by deformations in the object and or surface.

19: Define fluid friction.

Answer: Fluid friction describes the friction between layers within a viscous fluid that are moving relative to each other.

20: How frictions are reduced in machines?

Answer: Frictions are reduced in machines by using rolling in place of sliding; ball bearings are used for this purpose.

21: How can we reduce fluid friction?

Answer: The fluid friction can be minimised by giving suitable shapes to bodies moving in fluids.

22: Explain why sportsmen use shoes with spikes.

Answer: Sportsman use shoes with spike to increase the friction so that the shoes do not slip while they walk run or play.

23: A has to push a lighter box and B has to push same heavier box on the same floor, who will have to apply larger force and why?

Answer: B will have to apply larger force since; B will experience more frictional force because heavy object will be pressed hard against the opposite surface and produces more friction.

24: Why objects moving in fluid must have special shapes.

Answer: The fluids like various liquids or air that is; gaseous medium also exerts frictional force known as Drag. To minimize the effect of this Drag, streamlining of the motion of the object in the fluid is necessary which can be achieved by modification in the shape of the body. Objects moving in fluids must have a special shape called streamlined shapes or aerodynamic shapes. Streamlined Shape or Aerodynamic Shape is that shape which overcomes the friction of fluid.

25: Why it is difficult to walk on a floor wet with soapy water?

Answer: It is difficult to walk on a floor wet with soapy water because layer of soap makes floor smooth. The coating of soapy water reduces the friction and the foot cannot make a proper grip on the floor and it starts getting to slip on the floor.

26: Why it is difficult to walk on an extra smooth floor?

Answer: It is difficult to walk on a floor wet with soapy water because layer of soap makes floor smooth. The coating of soapy water reduces the friction and the foot cannot make a proper grip on the floor and it starts getting to slip on the floor.

27: Explain the function of spring balance.

Answer: Spring balance consists of a coiled spring which gets stretched when a force is applied to it, stretching of the spring is measured by a pointer moving on a graduated scale, the reading on the scale provides us with the magnitude of the force.

28: What is lubricated friction?

Answer: Fluid friction describes the friction between layers within a viscous fluid that are moving relative to each other.

29: What are the factors effecting friction?

Answer: A moving ball on the ground stops after some times because of the force of friction.

30: “Friction is an evil too”. Justify the statement.

Answer: Friction is an evil too because:

• It causes wear and tear in objects of our daily use including machines.
• It makes movement of heavy objects very difficult.
• We need to spend a lot of time and money for the maintenance of various machines in order to keep their friction low.

31: What is the cause of friction?

Answer: Friction is caused by the irregularities on the two surfaces in contact. The irregularities on the two surfaces lock into one another. Due to the interlocking of two surfaces friction arises.

32: Push the book on a table. You observe that after some time it stops. Explain why?

Answer: When we push a book on a table then a force acts on the book which opposes its motion. This force is called force of friction. The friction opposes the movement due to which the book stops.

33: What is a spring balance?

Answer: Spring balance is a device used for measuring the force acting on an object. It contains a coiled spring. The spring gets stretched when a force is applied on it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of force.

34: How does the friction get affected by the nature of surface?

Answer: Nature of surface is the major factor which affects the friction. When we attempt to move any surface, we have to apply a force to overcome the interlocking of the surfaces. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved.

35: Take two bricks, one is wrapped in jute while other is wrapped in polythene. Which one will be easier to drag and why?

Answer: It is easier to drag the brick which is wrapped in polythene. Polythene has a smooth surface. It has less friction than the jute. Thus, it requires lesser force to drag it. Jute has rough surface so it needs more force to drag it.

36: Why is it easy to drag a mat from the floor, but it is difficult to drag the mat when somebody is sitting on it?

Answer: We know that friction is caused by the interlocking of the irregularities in the two surfaces. It is obvious that the force of friction will increase, if the two surfaces are pressed harder. So it is easy to drag a mat when nobody is sitting on it but when a person is sitting on it then it produces more friction and not easy to drag it.

37: What are static and sliding frictions?

Answer: The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction.

38: Which is smaller, static friction or the sliding friction and why?

Answer: When the object starts sliding, the contact points on its surface, do not get enough time to lock into the contact points on the floor. So the sliding friction is slightly smaller than the static friction.

39: What happens, if the floor we walk on is frictionless?

Answer: We would not be able to walk on the surface if there is no friction on the floor. It is the friction which helps us to stand or walk on the surface. The grooves of our feet or shoes are locked into the irregularities of the floor and make us stand.

40: Why is it not easy to move an object from its static position?

Answer: When an object is at rest, it has better hold of the surface on which it is placed. In the static position the irregularities of the surface are interlocked properly due to which more force is required to overcome the friction. So it is  not easy to move an object from its static position.

41: Why can’t we write with a chalk if there were no friction?

Answer: We cannot write with a chalk, pen, pencil if there were no friction. If we are writing with a chalk on frictionless surface then no chalk particles stick to the surface. So we cannot write with a chalk if there were no friction. On the other hand when we are writing with a chalk on the blackboard, its rough surface rubs off some chalk particles which stick to the blackboard.

42: The soles of our shoes get worn out after a period of time. Explain why?

Answer: The soles of our shoes get worn out after a period of time due to friction. When we walk on the road, then the friction arises. Our shoes have to overcome friction to walk due to which our shoes get worn out slowly.

43: Our hands become warm when we rub them. Why?

Answer: When we rub our hands with each other the friction arises between two hands. The friction produces heat, this heat makes our hands warm. This is why our hands become warm when we rub them. We can feel this warmth by touching our hands on our face.

44: Mention three examples which show that friction produces heat.

Answer: Some examples which show that friction produces heat are following:
(i) Warming of our palms when we rub them.
(ii) Jar of a mixer becomes hot when it is run.
(iii) Warming of the parts of a machine when it is operated.

45: We observe that in some cases we want to increase friction. Explain why.

Answer: If we do not increase friction, vehicles will slip on the roads and they will never stop. We need to increase friction for better grip and to control the speed of the moving objects. It is not easy to walk and write without friction. We increase friction by making roads with concrete and by applying brakes in vehicles to stop them.

46: Why do we need to decrease friction and how it can be decreased?

Answer: Friction is harmful in some of the cases. It causes a lot of wear and tear in objects. It also causes damage to the machine parts. So we need to decrease the friction. It can be decreased by rolling, sliding and lubricating the surface in contact. We use ball bearings, wheels and lubricants like grease, oils and creams to decrease the friction.

47: How do wheels reduce friction?

Answer: We use wheels to reduce friction. The wheels reduce the resistance against motion. So it is easy to roll the body over one another than to slide a body. That is the reason that wheels reduce friction.

48: What are the factors on which frictional force depends in fluids?

Answer: There are following factors on which force of friction depends in fluids:
(i) The frictional force on an object in a fluid depends on its speed with respect to the fluid.
(ii) Frictional force also depends on the shape of the object and the nature of the fluid.

49: Why do we shape aeroplanes like that of a bird?

Answer: The objects are given special shapes to minimise friction. Aeroplane and birds both fly in the air and have to face friction exerted by air. They are so shaped that they do not have to lose energy while overcoming the frictional force exerted by air to fly. Therefore, the aeroplanes are also shaped streamlined, so that they can overcome the frictional force of air.

1: Explain why friction is a necessary evil?

Answer: Friction is very important for us. It allows us to walk comfortably, it allows us to grip and catch other objects, things do not move from their places because of the friction, speeds of moving objects can be controlled due to friction only.
But apart from this friction is considered as an evil because it causes wear and tear in objects of our daily use including machines. It makes movement of heavy objects very difficult. We need to spend a lot of time and money for the maintenance of various machines in order to keep their friction low.

2: Force of friction increases when two surfaces are pressed harder? Justify the statement.

Answer: Friction is caused by interlocking of irregularities in the two surfaces, thus it is quite obvious that the Force of friction increases when two surfaces are pressed harder, this can be experienced by dragging a mat when no one is sitting on it, and when a person is sitting on it.

3: Explain increasing and reducing friction.

Answer: Soles of shoes are grooved so that a better grip is provided to the surface and thus can move safely and freely on floors. The tyres of vehicles are treaded to provide better grip with the ground. Powder is sprinkled on the carom board to reduce friction and increase efficiency. Friction can never be entirely eliminated, because no surface is perfectly smooth, there are always some irregularities present in it.

Or

Explain in details why friction is called necessary evil.

(i) We cannot write with a pen or pencil, if there were no friction.
(ii) We cannot write on the blackboard with a chalk, if there were no friction.
(iii) If an object started moving, it would never stop, if there were no friction.
(iv) We cannot walk on the road without friction.
(v) We cannot fix a nail on the wall or tie a knot without friction.

(i) It wears out the materials whether they are screws, ball bearings or soles of shoes.
(ii) It causes damage to the parts of machines.
(iii) Friction also produces heat. When a machine is operated, heat is generated that causes wastage of energy.
(iv) The tyres of cars, buses and trucks, etc., also worn out due to friction.
(v) Friction reduces the speed due to which more force is required.
In this way, we see that friction is harmful as well as useful so it is called necessary evil.

5: Write some methods used to reduce friction.

(i) By using fine powder on the surfaces like on carrom board.
(ii) By using oils in the tools or in machine parts.
(iii) By using grease between the moving parts of the machines.
(iv) By using graphite powder between moving parts of a machine.
(v) By using air cushion between the moving parts in some machines where oils or lubricants may not be advisable.

6: Explain various types of frictions.

Answer: There are following types of frictions: (i) Static friction (ii) Sliding friction (iii) Rolling friction

(i) Static friction: The friction which is produced between two surfaces in contact with each other at the position of rest is called static friction. This type of friction is more powerful than others. It is not easy to overcome the static friction. This type of friction needs a lot of energy to overcome.

(ii) Sliding friction: The friction which arises when two objects in contact are in motion is called sliding friction. It is lesser than static friction but more than rolling friction. In this type of friction irregularities of surfaces do not get enough time lock with each other as objects are already in motion.

(iii) Rolling friction: The friction which arises when two bodies are rolling on each other is called rolling friction. Rolling movement of both objects do not allow the locking of irregularities, so it is the smallest friction out of three types of frictions.

7: What is fluid friction? Write the factors on which fluid friction depends.

Answer: The gases and liquids are called fluids. The friction exerted by fluids on an object is called fluid friction. The fluid friction is also called drag. The factors on which fluid friction depends are as following:
(i) The fluid friction of an object depends on its speed with respect to fluid.
(ii) The fluid friction of an object also depends on the shape of the object.
(iii) The frictional force due to fluids also depends on the nature of the fluid.