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Class 8 Science Chapter 9 Friction

Chapter 9 of Class 8 Science, titled "Friction," focuses on the force that opposes the relative motion or tendency of such motion of two surfaces in contact. The chapter explores the nature of friction, its causes, and the different types of friction, including static, sliding, and rolling friction. Students will learn about the factors that affect friction, such as the nature of the surfaces in contact and the force pressing them together. The chapter also highlights both the beneficial and harmful effects of friction in everyday life—how it helps in activities like walking or driving, and how it causes wear and tear in machines and engines. Moreover, the concept of reducing friction to improve efficiency in various mechanical systems, such as using lubricants and ball bearings, is also discussed. The quiz on this chapter will assess students’ understanding of the principles of friction, its real-world applications, and its effect on motion and energy. By the end of this chapter, students will gain a deeper understanding of how friction plays a crucial role in our daily activities and in the functioning of various technologies.

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Category: Introduction

1. Why is it difficult to walk on a smooth and wet floor?

2 / 100

Category: Introduction

2. If the sum of two numbers is 45 and one of them is 18, what is the other number?

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Category: Introduction

3. What is the force that opposes the relative motion between two surfaces in contact?

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Category: Observations of friction in daily life (slowing down vehicles, stopping objects, slipping on surfaces)

4. (A) When a book is pushed on a table, it eventually stops moving due to the force of friction.
(R) Friction acts in the opposite direction of the motion of the object.

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Category: Observations of friction in daily life (slowing down vehicles, stopping objects, slipping on surfaces)

5. (A) A moving ball on the ground stops eventually without any external force applied to it.
(R) Friction opposes the relative motion between the surfaces of the ball and the ground.

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Category: Observations of friction in daily life (slowing down vehicles, stopping objects, slipping on surfaces)

6. Why is it difficult to walk on a smooth and wet floor?

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Category: Understanding why friction occurs

7. Why does friction occur between two surfaces?

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Category: Understanding why friction occurs

8. (A) Static friction is greater than sliding friction.
(R) When an object starts moving, the contact points do not get enough time to interlock.

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Category: Understanding why friction occurs

9. What is the direction of the force of friction when an object is pushed to the left?

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Category: Force of Friction

10. What is the direction of the force of friction acting on a book when it is pushed to the right on a table?

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Category: Force of Friction

11. If a book stops moving after being pushed on a table, what force is responsible for bringing it to rest?

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Category: Force of Friction

12. (A) The force of friction always acts in the same direction as the applied force.
(R) The force of friction opposes the relative motion between the surfaces in contact.

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Category: Friction is a force that opposes motion

13. Which of the following factors does NOT affect the force of friction between two surfaces?

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Category: Friction is a force that opposes motion

14. (A) The force of friction between two surfaces depends on the roughness of the surfaces.
(R) Friction is caused by the interlocking of irregularities on the surfaces in contact.

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Category: Friction is a force that opposes motion

15. What is the primary role of friction between two surfaces in contact?

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Category: It acts between surfaces in contact

16. A block of mass $m$ is placed on a horizontal surface. The coefficient of friction between the block and the surface is $\mu$. If an additional weight $M$ is placed on top of the block, how does the frictional force change?

17 / 100

Category: It acts between surfaces in contact

17. A block is placed on a rough horizontal surface. A force $F$ is applied to the right, causing the block to move at a constant velocity. What can be inferred about the direction and magnitude of the frictional force acting on the block?

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Category: It acts between surfaces in contact

18. (A) The force of friction increases when two surfaces are pressed harder against each other.
(R) This is because the interlocking of irregularities between the surfaces becomes more pronounced with increased pressure.

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Category: Friction always acts in the direction opposite to motion

19. (A) When a book is pushed to the left on a table, the friction acts to the right.
(R) The force of friction always acts opposite to the direction of motion.

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Category: Friction always acts in the direction opposite to motion

20. A box weighing 100 N is pushed horizontally with a force of 40 N. If the coefficient of static friction between the box and the floor is 0.5, what is the maximum static friction force that can act on the box?

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Category: Friction always acts in the direction opposite to motion

21. (A) Friction always acts in the direction opposite to the motion of an object.
(R) The force of friction opposes the applied force, which causes the motion.

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Category: Factors Affecting Friction

22. A pencil cell is released from point A on an inclined plane covered with cloth. How does the distance it travels before coming to rest compare to when the plane is uncovered?

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Category: Factors Affecting Friction

23. (A) Sprinkling powder on a carrom board reduces friction.
(R) Powder acts as a lubricant, preventing direct contact between surfaces.

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Category: Factors Affecting Friction

24. Why do carrom players sprinkle fine powder on the carrom board?

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Category: More friction on rough surfaces due to increased interlocking

25. How does increasing the pressure between two surfaces affect the force of friction?

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Category: More friction on rough surfaces due to increased interlocking

26. How does increasing the pressure between two surfaces affect the force of friction?

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Category: More friction on rough surfaces due to increased interlocking

27. (A) The force of friction is greater on rough surfaces compared to smooth surfaces.
(R) Rough surfaces have a larger number of irregularities that interlock, increasing the friction.

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Category: Greater force required when surfaces are pressed harder

28. Which of the following statements correctly describes static friction?

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Category: Greater force required when surfaces are pressed harder

29. What is the relationship between static friction and sliding friction?

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Category: Greater force required when surfaces are pressed harder

30. A box is placed on a horizontal surface. When an additional weight is added to the box, the normal force increases by 50 N. How does the frictional force change if the coefficient of friction remains the same?

31 / 100

Category: Friction depends on surface texture and pressure

31. A brick is wrapped with a jute bag and pulled using a spring balance. The reading on the spring balance is noted. What would happen to the reading if the brick is wrapped with polythene instead of jute?

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Category: Friction depends on surface texture and pressure

32. (A) The force of friction increases when the surfaces in contact are rougher.
(R) The roughness of the surfaces causes more interlocking of irregularities, which increases friction.

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Category: Friction depends on surface texture and pressure

33. Which type of friction is generally smaller: sliding friction or static friction?

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Category: Friction: A Necessary Evil

34. What happens when you vigorously rub your palms together for a few minutes?

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Category: Friction: A Necessary Evil

35. (A) Friction is necessary for walking on a surface.
(R) Without friction, it would be impossible to walk as the feet would not grip the ground.

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Category: Friction: A Necessary Evil

36. (A) The force of friction increases when the surfaces in contact are pressed harder because the interlocking of irregularities becomes more pronounced.

(R) The increase in pressure between two surfaces reduces the irregularities, thereby increasing friction.

37 / 100

Category: Friction helps in walking, writing, and holding objects

37. (A) It is easier to hold a glass tumbler with a greasy surface than a dry one.
(R) Friction decreases when the surface becomes greasy, making it harder to hold objects.

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Category: Friction helps in walking, writing, and holding objects

38. Why is it difficult to walk on a wet marble floor?

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Category: Friction helps in walking, writing, and holding objects

39. Why is it harder to hold a greasy glass tumbler compared to a dry one?

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Category: Friction also causes wear and tear, and heat generation

40. What is one effect of friction on materials?

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Category: Friction also causes wear and tear, and heat generation

41. Which type of friction is required to overcome when an object starts moving from rest?

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Category: Friction also causes wear and tear, and heat generation

42. A machine is run for 30 minutes and due to friction, it generates heat energy of 1800 J. If the same machine is run for 45 minutes under the same conditions, what will be the heat energy generated?

43 / 100

Category: Motion of a pencil cell on different surfaces

43. If a pencil cell is wrapped with sandpaper and then rolled down an inclined plane covered with a smooth cloth, how does the distance travelled by the cell compare to when it is unwrapped and rolled down the same plane?

44 / 100

Category: Motion of a pencil cell on different surfaces

44. A pencil cell is rolled down an inclined plane on three different surfaces: a smooth table, a cloth-covered table, and a sand-covered table. On which surface will the pencil cell travel the least distance before coming to rest?

45 / 100

Category: Motion of a pencil cell on different surfaces

45. A pencil cell is released from a point on an inclined plane. It moves down the plane and then comes to rest on a table covered with a smooth cloth. If the same experiment is repeated with a sand-covered table, how does the distance covered by the pencil cell change?

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Category: Examples of useful friction: writing with chalk, gripping objects

46. Why is it easier to hold a kulhar (earthen pot) than a greasy glass tumbler?

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Category: Examples of useful friction: writing with chalk, gripping objects

47. Why is friction important when gripping objects like a glass tumbler?

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Category: Examples of useful friction: writing with chalk, gripping objects

48. When writing with chalk on a blackboard, why does the chalk leave a mark on the board?

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Category: Problems due to friction: worn-out shoes, overheating of machines

49. Which of the following is an example where friction produces heat?

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Category: Problems due to friction: worn-out shoes, overheating of machines

50. A car's brake pads generate heat when the brakes are applied. If the friction force between the brake pads and the wheel is 200 N and the car comes to a stop from a speed of 20 m/s in 5 seconds, how much heat is generated?

51 / 100

Category: Problems due to friction: worn-out shoes, overheating of machines

51. A person walks 10,000 steps every day on a rough surface. If each step causes a wear of 0.01 mm on the shoe sole, how much will the sole wear out in one year (365 days)?

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Category: Activity: Rubbing hands together to generate heat

52. What happens when you rub your palms together vigorously for a few minutes?

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Category: Activity: Rubbing hands together to generate heat

53. When you vigorously rub your palms together, why do they feel warm?

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Category: Activity: Rubbing hands together to generate heat

54. (A) Rubbing hands together generates heat due to friction.
(R) Friction converts mechanical energy into thermal energy.

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Category: Increasing and Reducing Friction

55. Why do brake pads in automobiles have a rough surface?

56 / 100

Category: Increasing and Reducing Friction

56. (A) Gymnasts apply a coarse substance on their hands to increase friction for better grip.
(R) Increasing friction helps in providing a better grip by enhancing the interlocking of irregularities between surfaces.

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Category: Increasing and Reducing Friction

57. (A) The soles of shoes are grooved to increase friction.
(R) Increasing friction provides better grip on the floor, ensuring safety.

58 / 100

Category: Increasing friction:

58. Kabaddi players often rub their hands with soil before a match. What is the primary reason for this action?

59 / 100

Category: Increasing friction:

59. (A) The soles of shoes are grooved to increase friction.
(R) Increased friction provides better grip on the floor.

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Category: Increasing friction:

60. (A) The soles of shoes are grooved to increase friction.
(R) Grooves on the soles provide a better grip on the floor, preventing slipping.

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Category: Grooved shoe soles and treaded tyres

61. (A) Grooved shoe soles and treaded tyres are designed to increase friction.
(R) The grooves and treads provide better grip by increasing the surface area in contact with the ground.

62 / 100

Category: Grooved shoe soles and treaded tyres

62. (A) Grooved shoe soles and treaded tyres are designed to increase friction by increasing the surface area in contact with the ground.
(R) The increased surface area enhances the interlocking of irregularities between the surfaces, thereby increasing friction.

63 / 100

Category: Brake pads in bicycles

63. What is the primary purpose of using brake pads in a bicycle?

64 / 100

Category: Brake pads in bicycles

64. What happens when brake pads press against the bicycle wheel rim?

65 / 100

Category: Kabaddi players using soil for grip

65. (A) Kabaddi players use soil to increase friction for a better grip.
(R) Soil increases the roughness of the hands, which enhances friction.

66 / 100

Category: Kabaddi players using soil for grip

66. In a bicycle brake system, the brake pads reduce the speed of the wheel by increasing friction. If the brake pads apply a normal force of 100 N and the coefficient of friction between the pads and the rim is 0.5, how much frictional force is generated to stop the wheel?

67 / 100

Category: Reducing friction:

67. Why is it easier to pull luggage fitted with rollers compared to sliding it?

68 / 100

Category: Reducing friction:

68. Why is it easier to pull luggage fitted with rollers compared to sliding it?

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Category: Lubricants like oil, grease, and graphite

69. (A) Grease is used as a lubricant in the moving parts of a bicycle to reduce friction.
(R) Grease forms a thin layer between the moving parts, preventing direct contact and reducing friction.

70 / 100

Category: Lubricants like oil, grease, and graphite

70. Which of the following is NOT a common lubricant?

71 / 100

Category: Powder on carrom boards

71. (A) Sprinkling fine powder on a carrom board reduces friction.
(R) The powder acts as a lubricant by forming a thin layer between the carrom pieces and the board.

72 / 100

Category: Powder on carrom boards

72. When oil is applied to the hinges of a door, the door moves smoothly. Which of the following principles is primarily responsible for this effect?

73 / 100

Category: Using air cushions in machines

73. In a machine using an air cushion, if the thickness of the air cushion is doubled while keeping the air pressure constant, how does this affect the friction between the moving parts?

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Category: Using air cushions in machines

74. What is a primary advantage of using air cushions over oil in reducing friction?

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Category: Wheels Reduce Friction

75. (A) The use of wheels in machinery reduces friction significantly because rolling friction is always less than sliding friction.
(R) Rolling friction occurs due to the deformation of the surfaces in contact, which minimizes the resistance to motion.

76 / 100

Category: Wheels Reduce Friction

76. Which of the following statements is true about rolling friction?

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Category: Rolling friction is much less than sliding friction

77. (A) Rolling friction is smaller than sliding friction because the contact area between the rolling object and the surface is reduced.
(R) Rolling reduces friction by converting sliding motion into rotational motion, which minimizes energy loss.

78 / 100

Category: Rolling friction is much less than sliding friction

78. Why are wheels considered one of the greatest inventions of mankind?

79 / 100

Category: Wheels and ball bearings reduce friction

79. A heavy metal block is moved across a rough surface using two different methods: sliding and rolling with the help of ball bearings. If the force required to slide the block is 100 N, what would be the approximate force required to roll it using ball bearings, given that rolling friction is 10\% of sliding friction?

80 / 100

Category: Wheels and ball bearings reduce friction

80. (A) It is easier to pull luggage fitted with rollers than to slide it.
(R) Rolling friction is smaller than sliding friction.

81 / 100

Category: Luggage with wheels

81. Which of the following is an example where rolling friction is utilized?

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Category: Luggage with wheels

82. Which of the following is an example where rolling friction is utilized to reduce effort?

83 / 100

Category: Rolling vs. sliding comparison with logs and machines

83. (A) The use of ball bearings in machines reduces friction because they convert sliding friction into rolling friction.
(R) Rolling friction is always less than sliding friction due to the reduced contact area and smoother motion.

84 / 100

Category: Rolling vs. sliding comparison with logs and machines

84. What is the purpose of using ball bearings in machines like ceiling fans and bicycles?

85 / 100

Category: Use of ball bearings in machines and vehicles

85. (A) The use of ball bearings in machines reduces friction because they convert sliding friction into rolling friction.
(R) Rolling friction is always less than sliding friction.

86 / 100

Category: Use of ball bearings in machines and vehicles

86. Why is rolling friction smaller than sliding friction?

87 / 100

Category: Fluid Friction (Drag)

87. What is the common name of gases and liquids in the context of friction?

88 / 100

Category: Fluid Friction (Drag)

88. (A) The shape of an aeroplane is designed to minimize fluid friction.
(R) Aeroplanes are given streamlined shapes inspired by the natural shapes of birds and fishes.

89 / 100

Category: Fluids (liquids and gases) also exert friction

89. A fish swims in water. Which of the following statements is true regarding the fluid friction experienced by the fish?

90 / 100

Category: Fluids (liquids and gases) also exert friction

90. An aeroplane is designed to have a shape similar to that of a bird. What is the primary reason for this design choice?

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Category: Fluid friction depends on speed, shape, and nature of the fluid

91. (A) The shape of an aeroplane is designed to reduce fluid friction.
(R) Special shapes help in minimising the energy lost due to fluid friction.

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Category: Fluid friction depends on speed, shape, and nature of the fluid

92. (A) The drag force on an object moving through a fluid is independent of the object’s speed.
(R) The shape of the object and the nature of the fluid are the only factors that determine the drag force.

93 / 100

Category: Drag: The frictional force in fluids

93. Which of the following factors does NOT directly affect the drag force experienced by an object moving through a fluid?

94 / 100

Category: Drag: The frictional force in fluids

94. (A) The drag force experienced by an object moving through a fluid can be minimized by shaping the object similarly to a bird or fish.
(R) Birds and fishes have evolved to have streamlined shapes that reduce energy loss due to fluid friction.

95 / 100

Category: Importance of streamlining shapes:

95. What is the frictional force exerted by fluids on objects moving through them called?

96 / 100

Category: Importance of streamlining shapes:

96. Which factor does NOT directly influence the drag force on an object moving through a fluid?

97 / 100

Category: Birds and fish have special body shapes

97. (A) Birds and fish have evolved streamlined body shapes to minimize energy loss due to fluid friction.
(R) The streamlined shape reduces the drag force acting on them by decreasing the surface area in contact with the fluid.

98 / 100

Category: Birds and fish have special body shapes

98. Which of the following shapes is most likely to reduce fluid friction?

99 / 100

Category: Aeroplanes and vehicles are designed to minimize fluid friction

99. Which of the following factors does NOT affect the drag force experienced by an object moving through a fluid?

100 / 100

Category: Aeroplanes and vehicles are designed to minimize fluid friction

100. Which of the following shapes would likely experience the least fluid friction when moving through air?

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