Friction Class 8 Science Chapter 12 Extra Questions and Answers are provided here. We prepared these extra questions based on the latest NCERT Class 8 Science Book. CBSE Class 8 Science Chapter 12 Friction Extra Questions will help you to properly understand a particular concept of the chapter.

Very Short Answer Type Question

Question 1: Which force helps things to move and stop?

Answer: Frictional force

Question 2: What enables a ladder to lean against the wall?

Answer: Friction

Question 3: Name two common lubricants.

Answer: Oil and Grease

Question 4: Mention one simple method of reducing friction between two surfaces.

Answer: Lubrication

Question 5: What is drag?

Answer: The frictional force exerted by fluids is also called drag.

Question 6: Which type of surface produces less friction?

Answer: Smooth surface produces less friction.

Question 7: Why the soles of our shoes wear out gradually?

Answer: The soles of shoes wear out gradually due to friction.

Question 8: Which type of surface produces more friction?

Answer: Rough surface produces more friction.

Question 9: Why ball bearings are used in machines?
Or
What is the purpose of using ball bearing in machine?

Answer: Ball bearings reduce friction. Thus, they are used in machines.

Question 10: Write any two machines in which ball bearings are used.

Answer: Ceiling fans and Bicycles

Question 11: What makes the steps of foot over-bridges at Railway Stations to wear out slowly?

Answer: Frictional force

Question 12: What are lubricants?

Answer: The substances which reduce friction are called lubricants.

Question 13: Why do we sprinkle fine powder on the carrom board?

Answer: Powder is sprinkled on the carrom board to reduce friction.

Question 14: Name the force responsible for wearing out of car tyres.

Answer: Frictional force is responsible the wearing of car tyres.

Question 15: What is the direction of force of friction acting on a moving object?
Or
What is the direction of force of friction?

Answer: The force of friction always opposes the applied force.

Question 16: Which force always opposes motion
Or
What is the force that always opposes motion?

Answer: Friction force always opposes motion.

Question 17: What is done to increase friction between the tyres and road?

Answer: The tyres of the vehicle are treaded to increase friction.

Question 18: When does static friction come into play?

Answer: Static friction comes into play when we try to move an object at rest.

Question 19: When does sliding friction come into play?

Answer: Sliding friction comes into play when an object is sliding over another.

Question 20: Why is the surface of a slide polished to make it smooth?

Answer: The surface of a slide polished to make it smooth in order to reduce friction.

Question 21: Why do kabaddi players rub their hands with dry soil?

Answer: Kabaddi players rub their hands with soil for a better grip of their opponents.

Question 22: Which type of friction comes into play when a book on cylindrical pencils is moved by pushing?

Answer: Rolling friction

Question 23: Name the device which is attached to heavy luggage (such as a heavy suitcase) to move it easily by pulling.

Answer: Rollers

Question 24: Give one example from everyday life where wheels (or rollers) are used to reduce friction.

Answer: Suitcase

Question 25: Why a bicycle and a motor mechanic use grease between the moving parts of these machines?

Answer: They do so to reduce friction in order to increase efficiency.

Question 26: Why are cars, aeroplanes and rockets streamlined?

Answer: Cars, aeroplanes and rockets are streamlined to reduce the fluid friction.

Question 27: What prevents us from slipping every time we take a step forward?

Answer: Frictional force prevents us from slipping every time we take a step forward.

Question 28: Why are treads made in the surface of tyres?

Answer: The treaded tyres of cars, trucks and bulldozers provide better grip with the ground.

Question 29: What are fluids?

Answer: In science, the common name of gases and liquids is fluids.

Question 30: Name two common fluids.

Answer: Two common fluids names are air and water.

Question 31: Why do gymnasts apply a coarse substance to their hands?

Answer: Gymnasts apply some coarse substance on their hands to increase friction for better grip.

Question 32: Why is it difficult to climb up an oily or greasy pole?

Answer: An oily or greasy pole has much less friction due to which it is difficult to climb up an oily or greasy pole.

Question 33: Which device is used between the hubs and axles of bicycle wheels to reduce friction?

Answer: Ball bearing is used between hubs and the axles of ceiling fans and bicycles to reduce friction.

Short Answer Type Questions

Question 1: Why do tyres of cars wear out gradually?

Answer: The tyres of cars wear out gradually due to friction between the surface of tyres and the surface of road.

Question 2: What is static friction?

Answer: The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction.

Question 3: What is sliding friction?

Answer: The force required to keep the object moving with the same speed is a measure of sliding friction.

Question 4: What is rolling friction?

Answer: When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.

Question 5: Why is it difficult to walk on a well-polished floor?

Answer: A well-polished floor offers very less resistance. Therefore, it is difficult to walk properly on a well-polished floor.

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

Answer: A smooth and wet floor offers very less resistance. Therefore, it is difficult to walk properly on a smooth and wet floor.

Question 7: Can we reduce friction to zero by polishing surfaces or using large amount of lubricants?

Answer: Friction can never be entirely eliminated. No surface is perfectly smooth. Some irregularities are always there.

Question 8: What happens when you rub your hands vigorously for a few seconds? Why does this happen?

Answer: If we rub our hands together for several seconds, then our hands feel warm. That warmth is caused by a force called friction.

Question 9: Explain why sportsmen use shoes with spikes.

Answer: Sportsmen use shoes with spikes because spikes increase the force of friction between the shoes and the ground and give them a better grip while running.

Question 10: Why do we slip when we step on a banana peel?

Answer: The inner side of banana peel being smooth and slippery reduces the friction between the sole of our shoes and the surface of road. Thus, we slip on it.

Question 11: Why the sole of our shoes is grooved?

Answer: The grooves are made in the soles of shoes to increase friction with the ground so that the shoes get a better grip on the floor and we can walk safely.

Question 12: Why do car wheels often spin on icy roads?

Answer: A car spins on icy road because the treads of the car can no longer maintain the proper friction to keep it moving. Due to ice, the friction reduces and the car spins.

Question 13: Why a pencil eraser loses tiny pieces of rubber each time we use it?

Answer: When we use a pencil eraser, friction between the eraser and the paper rubs off some rubber particles from the eraser. Thus, the eraser loses tiny pieces of rubber due to friction.

Question 14: Why a vehicle slows down when brakes are applied?

Answer: When brakes are applied, the brake pads press against the discs of the rotating car wheels. This produces friction between brake pads and the discs, making the wheels to slow down and ultimately stop.

Question 15: Why it is convenient to pull the luggages fitted with rollers?
Or
Even a child can pull attaches and other pieces of luggage fitted with rollers. Why is it so?

Answer: Rolling reduces friction. It is always easier to roll than to slide a body over another. That is the reason it is convenient to pull the luggages fitted with rollers.

Long Answer Type Questions

Question 1: What enables us to fix nails in a wall and knots to be tied?

Answer: When we hammer a nail in the wall, it is the friction between the surface of nail and wall which holds the nail tightly in the wall. Without friction, nails could not be fixed in a wall to hold things.Friction enables knots to be tied in the ropes.

Question 2: What is a spring balance?

Answer: Spring balance is a device used for measuring the force acting on an object. It 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 gives the magnitude of the force.

Question 3: How does a bicycle stop when its brakes are applied?

Answer: We deliberately increase friction by using brake pads in the brake system of bicycles and automobiles. When we are riding a bicycle, the brake pads do not touch the wheels. But when we press the brake lever, these pads arrest the motion of the rim due to friction. The wheel stops moving.

Question 4: When we strike a matchstick against the rough surface, it catches fire. Give reason.

Answer: When we strike a matchstick against the rough surface, then the friction between the head of the matchstick and rough surface produces heat. This heat burns the chemicals present on the head of the matchstick due to which the matchstick catches fire.

Question 5: Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Answer: When a book slides on the writing desk, a frictional force acts between the book and the surface of the desk. The direction of frictional force on the book is opposite to the direction of its motion and acts in upward direction.

Question 6: How lubricants work?

Answer: When oil, grease or graphite is applied between the moving parts of a machine, a thin layer is formed there and moving surfaces do not directly rub against each other. Interlocking of irregularities is avoided to a great extent. Thus, Movement becomes smooth.

Question 7: Explain why the sliding friction is less than the static friction.

Answer: Friction is caused by the interlocking of irregularities in the two surfaces. 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 and we find it somewhat easier to move the object already in motion than to get it started.

Question 8: Why will a pencil write on a paper, but not on a glass?

Answer: We are able to write on a paper because there is friction between the tip of the pencil and paper. We are not able to write on a glass because the glass surface is very smooth due to which friction between the tip of the pencil and glass surface is less. This friction is not enough to rub off black graphite particles from the tip of pencil.

Question 9: When we try to push a very heavy box kept on ground, it does not move at all. Which force is preventing this box to move forward? Where does this force act?

Answer: When we push a very heavy box kept on the ground, it does not move at all because frictional force balances the force that we apply. So, there is no net movement of this box. It acts in a direction opposite to the force we apply.

Question 10: Explain why objects moving in fluids must have special shapes.

Answer: Fluids exert force of friction on objects in motion through them. The frictional force on an object in a fluid depends on its speed with respect to the fluid. The frictional force also depends on the shape of the object and the nature of the fluid. It is obvious that when objects move through fluids, they have to overcome friction acting on them. In this process they lose energy. Efforts are, therefore, made to minimize friction.

Question 11: Give examples to show that friction is both a friend and a foe.

Answer: Advantages of friction:

• Friction enables us to walk.
• Friction enables us to write with pen or pencil.

 Disadvantages of friction:

• Friction wears away the soles of our shoes.
• The tyres of vehicles wear out gradually due to friction.

Question 12: You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?

Answer: We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction between the ground and feet decreases when there is soapy water spilled on the floor. Hence, it becomes difficult to walk on the soapy floor.

Question 13: Which is easier to hold in hand: a kulhar (earthen pot) or a glass tumbler? Why?

Answer: It is easier to hold a kulhar (earthen pot) in our hand than a glass tumbler. This is because due to rough surface of kulhar, the friction between kulhar and our hand is much more which makes it easier to hold it. On the other hand, due to the smooth surface of glass tumbler, the friction between glass tumbler and hand is much less which makes it comparatively difficult to hold it.

Question 14: 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: Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the floor, the interlocking of irregularities on the surfaces of box and floor become strong. This is because the two surfaces in contact are pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push the heavier box, Seema has to apply a greater force than Iqbal.

Question 15: How do rough and smooth surfaces affect friction?
Or
Why do smooth surfaces have less friction?
Or
Why do smooth surfaces have less friction than non-smooth surfaces?
Or
Why does a rough surface have greater friction than a smooth surface?
Or
Friction depends on the nature of two surfaces in contact. Explain.
Or
Is the friction the same for all the surfaces? Does it depend on the smoothness of the surfaces?

Answer: Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces which appear very smooth have a large number of minute irregularities on them. Irregularities on the two surfaces lock into one another. When we attempt to move any surface, we have to apply a force to overcome interlocking. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved.

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