Understanding Friction: Class 8 Science Questions and Answers
Very Short Answer Type Questions
1. Name the force which always opposes motion.
Answer: Friction
Explanation: Friction is a force that acts between two surfaces in contact and opposes their relative motion, making it harder for objects to move smoothly.
2. Why does a fast car slow down if its engine is switched off?
Answer: Due to friction.
Explanation: When the engine is off, no force propels the car forward. Friction between the tires and the road, along with air resistance, acts to slow down and eventually stop the car.
3. Which type of surfaces produce (a) least friction, and (b) too much friction?
Explanation: Smooth surfaces have fewer irregularities, reducing interlocking and thus friction. Rough surfaces have more irregularities, increasing friction due to greater interlocking.
4. What is the direction of force of friction acting on a moving object?
Answer: Opposite to the direction of motion.
Explanation: Friction always acts to oppose the relative motion between two surfaces, so it is directed opposite to the object’s movement.
5. Name a device which is used to measure force acting on an object.
Answer: Dynamometer
Explanation: A dynamometer measures the force applied to an object, often used in experiments to quantify forces like friction or tension.
6. What is a spring balance?
Answer: A device to measure weight or force.
Explanation: A spring balance measures the force (weight) of an object by the extension or compression of a spring, calibrated to show force in newtons or weight in grams.
7. Among sliding friction, static friction, and rolling friction: (a) which one is the smallest? (b) which one is the largest?
Answer: (a) Rolling friction is the smallest. (b) Static friction is the largest.
Explanation: Rolling friction is the least because rolling reduces surface contact and interlocking. Static friction is the largest as it requires the most force to initiate motion between stationary surfaces.
8. Which type of friction comes into play when a book kept on cylindrical pencils is moved by pushing?
Answer: Rolling friction
Explanation: The cylindrical pencils act like rollers, allowing the book to roll rather than slide, so rolling friction is involved.
9. Why is it more difficult to walk properly on a well-polished floor?
Answer: Reduced friction.
Explanation: A well-polished floor is smooth, reducing friction between the floor and shoe soles, making it harder to grip and increasing the chance of slipping.
10. Why is it difficult to walk on a wet marble floor?
Answer: Low friction due to water.
Explanation: Water acts as a lubricant, reducing friction between the shoe soles and the marble floor, making it slippery and difficult to walk without slipping.
11. Which force is responsible for the wearing out of car tyres?
Answer: Friction
Explanation: Friction between the tyres and the road causes wear and tear as the tyre surface rubs against the rough road, gradually eroding the tread.
12. What prevents you from slipping every time you take a step forward?
Answer: Friction
Explanation: Friction between your shoe soles and the ground provides the grip needed to push forward without slipping, enabling stable walking.
13. Name the force which helps things to move and stop.
Answer: Friction
Explanation: Friction allows objects to gain traction to move (e.g., walking or driving) and also slows them down or stops them when the driving force ceases.
14. What enables us to fix nails in a wall and knots to be tied?
Answer: Friction
Explanation: Friction holds nails in place by resisting their removal from the wall and keeps knots tight by preventing the rope from slipping.
15. What makes the steps of foot over-bridges at railway stations wear out slowly?
Answer: Friction
Explanation: Constant walking on the steps causes friction between shoe soles and the step surface, gradually wearing out the material over time.
16. What is done to increase friction between the tyres and road?
Answer: Adding treads to tyres.
Explanation: Treads (patterns on tyres) increase surface roughness, enhancing grip and friction with the road, reducing the risk of skidding.
17. Why do gymnasts apply a coarse substance to their hands?
Answer: To increase friction.
Explanation: Coarse substances like chalk increase friction between hands and equipment (e.g., bars), improving grip and preventing slipping during routines.
18. Why do kabaddi players rub their hands with dry soil?
Answer: To increase friction.
Explanation: Dry soil roughens the hands, increasing friction and allowing players to grip opponents better during the game.
19. Name the device which is used between the hubs and axles of bicycle wheels to reduce friction.
Answer: Ball bearings
Explanation: Ball bearings reduce friction by allowing rolling motion between the hub and axle, minimizing surface contact and resistance.
20. What is the purpose of using ball bearings in machines?
Answer: To reduce friction.
Explanation: Ball bearings convert sliding friction into rolling friction, which is much lower, allowing smoother and more efficient machine operation.
21. Name any two machines in which ball bearings are used.
Answer: Bicycles and ceiling fans.
Explanation: Ball bearings are used in bicycle wheels to reduce friction during rotation and in ceiling fans to ensure smooth blade movement.
22. Name the device which is attached to heavy luggage to move it easily by pulling.
Answer: Wheels
Explanation: Wheels (often on a trolley or suitcase) reduce friction by allowing rolling motion, making it easier to pull heavy luggage.
23. Name one example from everyday life where wheels (or rollers) are used to reduce friction.
Answer: Shopping trolley
Explanation: Wheels on a shopping trolley reduce friction, allowing easy movement of heavy loads with minimal effort.
24. Why does oiling the axles of a bicycle make the bicycle move more easily?
Answer: It reduces friction.
Explanation: Oil acts as a lubricant, reducing friction between the axle and wheel hub, allowing smoother rotation and easier movement.
25. State one way in which the friction between wheel and its axle can be reduced.
Answer: Using ball bearings.
Explanation: Ball bearings reduce friction by replacing sliding motion with rolling motion, which requires less force to overcome.
26. Name two common lubricants.
Answer: Oil and grease.
Explanation: Oil and grease are lubricants that reduce friction between surfaces by creating a smooth layer, commonly used in machines and vehicles.
27. Why do we sprinkle fine powder on a carrom board?
Answer: To reduce friction.
Explanation: Fine powder (like talcum) reduces friction between the striker and the board, allowing smoother and faster movement during the game.
28. Which force gets reduced when the two surfaces in contact are polished to make them smooth?
Answer: Friction
Explanation: Polishing reduces surface irregularities, decreasing interlocking between surfaces and thus reducing the frictional force.
Short Answer Type Questions
36. When we try to push a very heavy box kept on the ground, it does not move at all. Which force is preventing this box from moving forward? Where does this force act?
Answer: Static friction prevents the box from moving, acting at the contact surface between the box and the ground.
Explanation: Static friction opposes the applied force, preventing motion until the applied force exceeds the maximum static friction. It acts parallel to the surface, opposite to the push.
37. Suppose your writing table (or desk) is tilted a little. A book kept on the table starts sliding down. Draw a diagram to show the direction of force of friction acting on the book.
Answer: The frictional force acts upward along the incline, opposite to the downward sliding motion of the book.
Explanation: As the book slides down due to gravity, friction opposes the motion, acting upward along the tilted surface to slow or stop the book’s descent.
38. Which will cause more friction: a rough surface or a smooth surface? Why?
Answer: A rough surface causes more friction.
Explanation: Rough surfaces have more irregularities, leading to greater interlocking between surfaces, which increases friction compared to smooth surfaces with less interlocking.
39. Explain why sliding friction is less than static friction.
Answer: Sliding friction is less because moving surfaces have less interlocking time.
Explanation: Static friction requires overcoming the initial interlocking of surface irregularities, needing more force. Once motion starts, sliding friction involves less contact time, reducing friction.
40. What is meant by 'rolling friction'?
Answer: Rolling friction is the force opposing the motion of a rolling object.
Explanation: It occurs when an object like a wheel rolls over a surface, involving less resistance than sliding due to minimal surface contact and deformation.
41. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer: Seema will apply a larger force.
Explanation: Friction depends on the normal force, which increases with weight. The heavier box has greater friction, requiring more force to overcome it.
42. Why does a man slip when he steps on a banana peel thrown on the road?
Answer: Reduced friction causes slipping.
Explanation: The banana peel acts as a lubricant, reducing friction between the shoe and the ground, making it hard to maintain grip and causing a slip.
43. Car wheels often spin on icy roads. Explain why.
Answer: Low friction due to ice.
Explanation: Ice reduces friction between tyres and the road, causing the wheels to spin without sufficient grip to propel the car forward effectively.
44. Explain why: (a) a pencil will write on paper but not on glass.
Answer: Paper provides more friction.
Explanation: Paper’s rough surface creates enough friction to allow graphite to stick when a pencil is rubbed, while glass’s smooth surface offers little friction, preventing graphite deposition.
44. (b) Climbing a greasy pole is very difficult.
Answer: Grease reduces friction.
Explanation: Grease acts as a lubricant, lowering friction between hands/feet and the pole, making it hard to grip and climb without slipping.
45. Why does a matchstick light when we strike it on a rough surface?
Answer: Friction produces heat.
Explanation: Striking the matchstick on a rough surface generates friction, producing enough heat to ignite the chemical coating on the match head.
46. Why is it difficult to light a matchstick by striking it on a smooth surface?
Answer: Smooth surfaces produce less friction.
Explanation: A smooth surface offers minimal friction, generating insufficient heat to ignite the matchstick’s chemical coating.
47. Which parts wear away first in shoes? Give a reason for your answer.
Answer: The heels wear out first.
Explanation: Heels experience more friction due to greater pressure and contact with the ground during walking, causing faster wear compared to other parts.
48. Why do brake pads of bicycles have to be replaced quite often?
Answer: Friction causes wear.
Explanation: Brake pads rub against the wheel rim to stop the bicycle, and this constant friction wears them down, necessitating frequent replacement.
49. A pencil eraser loses tiny pieces of rubber each time you use it. Why does this happen?
Answer: Friction causes wear.
Explanation: Rubbing the eraser on paper generates friction, which breaks off small rubber particles due to the abrasive interaction with the paper’s surface.
50. What happens when you rub your hands vigorously for a few seconds? Why does this happen?
Answer: Hands become warm due to friction.
Explanation: Rubbing hands creates friction between the skin surfaces, converting kinetic energy into heat, warming the hands.
51. Explain how friction enables us to walk without slipping.
Answer: Friction provides grip.
Explanation: When walking, you push backward against the ground. Friction opposes this, providing a forward reaction force, allowing you to move without slipping.
52. Which is easier to hold in hand: a kulhar (earthen pot) or a glass tumbler? Why?
Answer: Kulhar is easier to hold.
Explanation: The kulhar’s rough earthen surface provides more friction, improving grip, while the smooth glass tumbler offers less friction, making it harder to hold.
53. How does a bicycle stop when its brakes are applied?
Answer: Friction from brakes slows the wheel.
Explanation: Brakes press pads against the wheel rim, creating friction that opposes the wheel’s rotation, slowing and stopping the bicycle.
54. Explain why the soles of our shoes wear out gradually.
Answer: Friction with the ground causes wear.
Explanation: Constant rubbing of shoe soles against the ground during walking generates friction, eroding the sole material over time.
55. Why do tyres of cars wear out gradually?
Answer: Friction with the road causes wear.
Explanation: Tyres rub against the road while driving, and this friction wears down the tread, gradually reducing the tyre’s thickness.
56. State two advantages and two disadvantages of friction.
Answer:
Advantages:
- Enables walking by providing grip.
- Allows vehicles to move and stop.
Disadvantages:
- Causes wear and tear of materials.
- Reduces efficiency by opposing motion.
Explanation: Friction helps in daily activities like walking and driving by providing necessary grip, but it also causes material degradation and energy loss in machines.
57. Explain why sportsmen use shoes with spikes.
Answer: Spikes increase friction.
hedging their bets on the ground, increasing grip and preventing slipping during high-speed movements.
58. How will you reduce friction between those machine parts which rub against each other? Give the simplest method.
Answer: Apply a lubricant like oil.
Explanation: Lubricants create a thin layer between moving parts, reducing direct contact and friction, thus minimizing wear and tear.
59. What is meant by lubrication? Why is it important?
Answer: Lubrication is the application of substances to reduce friction.
Explanation: It’s important because it minimizes friction, reduces wear, prevents overheating, and extends the lifespan of machine parts.
60. Explain why wheels are so useful.
Answer: Wheels reduce friction, enabling easy movement.
Explanation: Wheels convert sliding friction into rolling friction, which is much lower, making it easier to transport heavy loads with less effort.
61. Why are lubricants (oil or grease) applied to rubbing surfaces of machines?
Answer: To reduce friction and wear.
Explanation: Lubricants form a protective layer, reducing direct contact between surfaces, minimizing friction, heat, and damage to machine parts.
62. Explain with the help of diagrams, how the use of oil reduces friction between two surfaces in contact with each other.
Answer: Oil creates a smooth layer between surfaces.
Explanation: Without oil, surface irregularities interlock, increasing friction. Oil fills gaps, reducing direct contact and allowing smoother relative motion, lowering friction.
63. Why are cars, aeroplanes, and rockets streamlined?
Answer: To reduce air resistance (drag).
Explanation: Streamlined shapes minimize air turbulence and resistance, improving speed and fuel efficiency by reducing the drag force.
64. Explain why a speedboat has a streamlined shape.
Answer: To reduce water resistance.
Explanation: A streamlined shape cuts through water efficiently, reducing drag and allowing the speedboat to move faster with less energy.
65. What are fluids? Name two common fluids.
Answer: Fluids are substances that flow; examples are water and air.
Explanation: Fluids (liquids and gases) lack a fixed shape and flow under force, like water in a river or air in the atmosphere.
Long Answer Type Questions
66. (a) Define friction. What are the factors affecting friction? Explain with examples.
Answer: Friction is the force opposing relative motion between two surfaces in contact.
Factors:
- Surface roughness: Rough surfaces (e.g., sandpaper) produce more friction than smooth ones (e.g., ice).
- Normal force: Greater force pressing surfaces together (e.g., a heavy box) increases friction.
Explanation: Friction depends on surface texture and the force between them, affecting how easily objects move.
66. (b) What is the cause of friction? Explain with the help of a labelled diagram.
Answer: Friction is caused by interlocking surface irregularities.
Explanation: Microscopic bumps on surfaces interlock, resisting motion. A diagram would show two rough surfaces with interlocking protrusions, causing resistance when one slides over the other.
67. (a) Give examples to show that friction depends on the nature of two surfaces in contact.
Answer:
- Rubbing sandpaper on wood creates high friction due to roughness.
- Sliding on ice produces low friction due to smoothness.
Explanation: The degree of roughness or smoothness directly affects the amount of friction between surfaces.
67. (b) Give an example to show that friction depends on the force with which the two surfaces are pressed together.
Answer: Pushing a heavy box requires more force than a light one.
Explanation: A heavier box presses harder against the ground, increasing the normal force and thus friction, making it harder to move.
68. (a) What is the difference between static friction and sliding friction? For a given pair of objects, which of the two is greater?
Answer: Static friction prevents motion; sliding friction opposes moving objects. Static friction is greater.
Explanation: Static friction requires more force to initiate motion due to stronger interlocking, while sliding friction is lower as surfaces are already in motion.
68. (b) How can a very heavy machine be moved conveniently from one place to another in a factory? (No crane is available for this purpose).
Answer: Use rollers or wheels.
Explanation: Rollers or wheels reduce friction by enabling rolling motion, making it easier to move heavy machines with less force.
69. (a) What is drag? Give two examples of a drag force.
Answer: Drag is the frictional force exerted by fluids.
Examples:
- Air resistance on a moving car.
- Water resistance on a swimming fish.
Explanation: Drag opposes motion through fluids like air or water, slowing down objects.
69. (b) How can you reduce the drag on something moving through the air?
Answer: Use a streamlined shape.
Explanation: Streamlined shapes reduce air turbulence, minimizing drag and allowing smoother, faster movement through the air.
70. (a) What is meant by 'streamlined shape'? Name an object which usually has a streamlined shape.
Answer: A streamlined shape is narrow at the ends and wider in the middle; example: an aeroplane.
Explanation: This shape reduces fluid resistance, enabling efficient movement through air or water.
70. (b) Explain why objects moving in fluids should have a streamlined shape.
Answer: To reduce drag.
Explanation: Streamlined shapes minimize resistance from fluids, improving speed and efficiency by reducing turbulence and drag forces.
Multiple Choice Questions (MCQs)
71. A boy runs his toy car on dry marble floor, wet marble floor, newspaper, and towel spread on the floor. The force of friction acting on the car in increasing order will be:
Options:
(a) Wet marble floor, Dry marble floor, Newspaper, Towel [Correct]
(b) Newspaper, Towel, Dry marble floor, Wet marble floor
(c) Towel, Newspaper, Dry marble floor, Wet marble floor
(d) Wet marble floor, Dry marble floor, Towel, Newspaper
Explanation: Wet marble has the least friction due to water acting as a lubricant, followed by dry marble, newspaper, and towel, which is rougher.
72. Four children arrange forces due to rolling, static, and sliding friction in decreasing order. The correct arrangement is:
Options:
(a) Rolling, Static, Sliding
(b) Rolling, Sliding, Static
(c) Static, Sliding, Rolling [Correct]
(d) Sliding, Static, Rolling
Explanation: Static friction is the highest, followed by sliding, then rolling, which is the least due to minimal surface contact.
73. A big wooden box is being pushed from east to west. The force of friction due to the ground will act towards:
Options:
(a) north direction
(b) south direction
(c) east direction [Correct]
(d) west direction
Explanation: Friction opposes the motion, acting in the opposite direction (east) to the box’s westward movement.
74. A spring balance can be used to measure:
Options:
A. Mass of an object
B. Force acting on an object
C. Density of an object
D. Weight of an object
(a) A and B
(b) B and C
(c) B and D [Correct]
(d) only D
Explanation: A spring balance measures force (B) and weight (D), which is the force due to gravity.
75. The friction between two surfaces does not depend on one of the following:
Options:
(a) amount of surface area in contact [Correct]
(b) weight of the object
(c) degree of smoothness
(d) degree of roughness
Explanation: Friction depends on normal force and surface nature, not the contact area for most surfaces.
76. If the sliding friction between two surfaces is 8 N, then the static friction is most likely to be:
Options:
(a) 5 N
(b) 10 N [Correct]
(c) 4 N
(d) 2 N
Explanation: Static friction is typically higher than sliding friction, so 10 N is a likely value.
77. Which of the following is not an advantage of friction?
Options:
(a) Enables drawing on paper
(b) Enables picking up objects
(c) Enables rubber pads to be rubbed off [Correct]
(d) Enables vehicles to move
Explanation: Rubbing off rubber pads is a disadvantage, as it causes wear, unlike the other options, which are beneficial.
78. Which of the following statements is incorrect?
Options:
(a) Static friction is greater than rolling friction
(b) Sliding friction is less than rolling friction [Correct]
(c) Rolling friction is less than static friction
(d) Static friction is greater than sliding friction
Explanation: Sliding friction is greater than rolling friction, so option (b) is incorrect.
79. If the static friction between two surfaces is 20 N, then the rolling friction is most likely to be:
Options:
(a) 25 N
(b) 20 N
(c) 5 N [Correct]
(d) 50 N
Explanation: Rolling friction is much lower than static friction, making 5 N a reasonable estimate.
80. If the static friction between two surfaces is 50 N, then the sliding friction is most likely to be:
Options:
(a) 75 N
(b) 45 N [Correct]
(c) 5 N
(d) 65 N
Explanation: Sliding friction is less than static friction, so 45 N is a likely value.
81. Which of the following will produce the maximum friction?
Options:
(a) Rubbing sandpaper on glazed paper
(b) Rubbing sandpaper on glass tabletop
(c) Rubbing sandpaper on aluminium frame
(d) Rubbing sandpaper on sandpaper [Correct]
Explanation: Two rough surfaces (sandpaper on sandpaper) produce the maximum friction due to extensive interlocking.
82. Four similar cars stop after covering distances of 5 m, 5.5 m, 4.8 m, and 5.2 m. The friction between the brake pads and discs will be the maximum in the car which travels:
Options:
(a) 5 m
(b) 5.5 m
(c) 4.8 m [Correct]
(d) 5.2 m
Explanation: The car stopping in the shortest distance (4.8 m) experiences the highest friction, as greater friction reduces stopping distance.
83. The weight of an object can be measured by a:
Options:
(a) beam balance
(b) analytical balance
(c) spring balance [Correct]
(d) physical balance
Explanation: A spring balance measures weight (force due to gravity), unlike other balances that measure mass.
84. A book on a tilted table slides down slowly because:
Options:
(a) Sliding friction is greater than static friction
(b) Sliding friction is less than the force of gravity [Correct]
(c) Static friction is greater than sliding friction
(d) Force of gravity is less than sliding friction
Explanation: The book slides when the gravitational force exceeds sliding friction, which is less than static friction.
87. Which of the following does not have a streamlined shape?
Options:
(a) Aeroplane
(b) Boat
(c) Bird
(d) Bus [Correct]
Explanation: Buses are not streamlined, unlike aeroplanes, boats, and birds, which are designed to reduce fluid resistance.
88. The frictional force exerted by a fluid is called:
Options:
(a) Drag [Correct]
(b) Drab
(c) Drag
(d) Tread
Explanation: Drag is the term for frictional resistance in fluids like air or water. (Note: Option (c) appears to be a duplicate of (a) in the document.)
89. A person with mustard oil on their hands will find it most difficult to hold:
Options:
(a) Earthen cup (kulhar)
(b) Thermocol tumbler
(c) Glass tumbler [Correct]
(d) Wooden cup
Explanation: Oil reduces friction, and a smooth glass tumbler offers the least grip, making it the hardest to hold.
90. Ball bearing usually converts:
Options:
(a) Rolling friction into sliding friction
(b) Static friction into sliding friction
(c) Sliding friction into rolling friction [Correct]
(d) Rolling friction into static friction
Explanation: Ball bearings reduce friction by converting sliding motion into rolling motion, which has less resistance.
High Order Thinking Skills (HOTS) Questions
91. A pencil cell travels 35 cm on floor A and 20 cm on floor B before stopping. Which floor offers greater friction?
Answer: Floor B offers greater friction.
Explanation: The pencil cell stops in a shorter distance (20 cm) on floor B, indicating higher friction, as greater friction slows the object faster.
92. A car is moving towards North. What will be the direction of force of friction acting on this car due to the surface of the road?
Answer: South
Explanation: Friction opposes the car’s northward motion, so the frictional force acts in the opposite direction, towards the south.
93. You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer: More difficult.
Explanation: Soapy water reduces friction by acting as a lubricant, making the floor slippery and increasing the risk of slipping while walking.
94. What kind of friction comes into play: (a) when a block of wood moves slowly on a table? (b) when a block of wood just tends to move? (c) when a block of wood moves on cylindrical iron rods?
Answer:
(a) Sliding friction
(b) Static friction
(c) Rolling friction
Explanation:
(a) The block slides, so sliding friction opposes the motion.
(b) The block is about to move, so static friction prevents motion.
(c) Cylindrical rods enable rolling, so rolling friction is involved.
95. Explain why it is easier to drag a mat on the floor when nobody is sitting on it but much more difficult when a person is sitting on it.
Answer: Increased weight increases friction.
Explanation: A person’s weight increases the normal force, which increases friction, making it harder to drag the mat compared to when it’s empty.
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