Living Science 2019 2020 Solutions for Class 8 Science Chapter 12 Friction are provided here with simple step-by-step explanations. These solutions for Friction are extremely popular among Class 8 students for Science Friction Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science 2019 2020 Book of Class 8 Science Chapter 12 are provided here for you for free. You will also love the ad-free experience on Meritnationâ€™s Living Science 2019 2020 Solutions. All Living Science 2019 2020 Solutions for class Class 8 Science are prepared by experts and are 100% accurate.

#### Question 1:

Does friction always oppose motion?

No, friction does not always oppose the motion. When we walk on the ground, we push the ground in the backward direction. Then it is the friction force between the ground and us, which helps us moving forward. Also, while taking a turn on a circular road, friction force works as a centripetal force and helps in changing the direction.

#### Question 2:

A surface is rubbed to make it smoother. What effect will this have on friction? Give reasons.

The friction force is basically due to the irregularities of surfaces. If a surface is rubbed to make it smoother, then the force of the friction provided by that surface will reduce. A perfectly smooth surface offers negligible friction.

#### Question 3:

Two surfaces are rubbed together, first with a small force and then with a greater force. In which case will the friction between the surfaces be higher? Give reasons.

Friction force opposes the relative motion between two surfaces in contact. If two surfaces are rubbed with a small force then simply the opposing force would also be small, but if they are rubbed with a stronger force then friction force will try to oppose the force involved. Hence, it would be higher.

#### Question 1:

Since friction slows us down, we can run very fast if friction is eliminated. Do you agree? Give reasons.

No, I do not agree. Friction force slows us down but it also helps us in moving. If the friction force would be eliminated then we won't be able to walk or run on any surface. For example, when we walk or run on the ground, we push the ground in a backward direction. As a reaction, the ground pushes us back to move forward. That reaction force of ground is nothing but friction force.

#### Question 2:

It was found that a machine with moving parts was getting overheated. What was the heat due to? How can the heat generated be reduced?

The heat produced is due to the friction force. The surfaces of the moving parts of the machine must have friction. In order to move against each other, they have to overcome the force of friction. The work done by the moving parts against the frictional forces between their surfaces appears as heat. Generally, oil lubrication is applied in the parts of the machine which makes their surfaces smoother and hence reduces the friction between them. As a result, less heat is produced.

#### Question 3:

Does air exert friction on an object moving through it?

Air also opposes the motion of the object moving through it. The resistance force exerted by air is commonly known as the 'Air drag'.

#### Question 1:

Friction is
(a) always a disadvantage.
(b) always an advantage.
(c) sometimes a disadvantage and sometimes an advantage.
(d) neither a disadvantage nor an advantage.

(c) sometimes a disadvantage and sometimes an advantage
Friction is sometimes a disadvantage and sometimes an advantage.

#### Question 2:

It is difficult to walk on ice because
(a) pressure is high.
(b) pressure is low.
(c) friction is high.
(d) friction is low.

(d) friction is low
Ice offers less friction.

#### Question 3:

Friction can be increased by
(a) making the surfaces smooth.
(b) lubricating the surfaces.
(c) using ball bearings.
(d) making the surfaces rough.

(d) making the surfaces rough

Friction increases with roughness.

#### Question 4:

Which of these is true about friction?
(a) It can stop a moving object.
(b) It can change the shape of an object.
(c) It can change the direction of a moving object.
(d) It can make a moving object move faster.

(a) It can stop a moving object.
Friction acts as a decelerating force against motion; it helps in slowing down a body, thereby eventually bringing it to rest.

#### Question 5:

In which case is friction a disadvantage?
(a) running a machine
(b) walking
(c) applying brakes
(d) writing

(a) Running a machine

While running a machine, energy is lost because of friction. Hence, in this case, friction is a disadvantage.

#### Question 6:

Ball bearings are useful because
(a) rolling friction is more than sliding friction.
(b) rolling friction is less than siliding friction.
(c) rolling friction is same as sliding friction.
(d) it is easier to apply grease to ball bearings to reduce friction.

(b) rolling friction is less than sliding friction

Rolling friction is less than sliding friction because of the minimal surface of contact between two bodies.

#### Question 1:

Friction always opposes __________

Friction always opposes the motion of a body.

#### Question 2:

Friction is caused by __________ on the surfaces in contact.

Friction is caused by irregularities on the surfaces in contact.

#### Question 3:

Is friction a contact or a non-contact force?

Friction is a contact force, i.e., it requires two bodies to be in contact.

#### Question 4:

The smoother a surface is, the __________ (greater/lesser) will be the friction.

lesser

Frictional force decreases with the decrease in the roughness of the surfaces in contact.

#### Question 5:

Brakes on cars will work best if the friction between the brake shoes and wheels is reduced. True or false?

False. Frictional force helps in reducing the motion of a body. More the friction, more the opposing force and faster the stoppage of the car.

#### Question 6:

Friction causes wastage of energy. True or false?

True. Friction causes loss of energy in the form of heat, sound, light, etc.

#### Question 7:

A machine with moving parts will become less hot while running if it is well lubricated. True or false?

True
Friction causes loss of energy in the form of heat. When a machine is well lubricated, friction is reduced; therefore, less heat is produced.

#### Question 8:

Friction due to fluids is called __________

Friction due to fluids is called drag.

#### Question 9:

Rolling friction is __________ (greater/smaller) than sliding friction.

Rolling friction is smaller than sliding friction.

#### Question 10:

To reduce fluid friction, the object should have a __________ shape.

To reduce fluid friction, the object should have a streamlined shape.

#### Question 1:

What is friction?

Friction is a force that resists the relative motion between two surfaces in contact. It is a contact force.

#### Question 2:

An object is moving from north to south. What is the direction of the force of friction on the object?

Frictional force acts in the direction opposite to the direction of motion of a body. Because the object is moving from north to south, the direction of frictional force is from south to north.

#### Question 3:

On what principle do ball bearings work?

A ball bearing works on the principle that rolling friction is less than sliding friction. It helps in reducing friction between two surfaces in contact and maintaining separation between the bearing races, thereby allowing minimal surface contact between two surfaces.

#### Question 4:

How does lubrication reduce friction?

Lubrication helps in reducing roughness between two surfaces in contact. Thus, friction decreases with the reduction in the roughness between two surfaces.

#### Question 5:

Give an example to show the effect of heat generated in a machine due to excessive friction.

Friction opposes the relative motion between two surfaces in contact. The loss of energy occurs in the form of heat, sound, light, etc. For example, mechanical engines are lubricated time to time to reduce excessive friction that prevents the loss of energy in the form of heat. Take example of a car, motorbike or bicycle whose mechanical parts are lubricated to reduce excessive friction.

#### Question 1:

What is the effect of friction on motion?

Friction is a force that reduces the relative motion between two surfaces in contact. It helps in reducing the motion of a body and to slow down the body till it stops.

#### Question 2:

What causes friction between two surfaces in contact?

The roughness of surfaces causes friction between two surfaces in contact. To ordinary eyes, the surfaces may seem smooth, but at a microscopic level, the surfaces have crest and trough. Under magnification, they look a little like mountain ranges, full of peaks and valleys. Hence, when two surfaces come in contact, the motion between them is reduced because of the crest and trough on the surfaces.

#### Question 3:

Why does a rough surface have greater friction than a smooth surface?

Rough surfaces have greater friction than smooth surfaces. Rough surfaces have grooves on them. When two rough surfaces are in contact, the grooves of one go inside the grooves of the other, thus making the movement difficult. Hence, roughness increases friction.

#### Question 4:

Give two situations where it is desirable to increase friction.

Two situations where it is desirable to increase friction:
1) Walking - Friction is required between feet/footwear and the ground to prevent slips.
2) Machine Brakes - Friction is desirable in brakes to reduce motion in a quick time.

#### Question 5:

What is 'streamlining'? How is it useful?

Streamlining is a phenomenon through which objects are designed in such a way that a fluid passes easily over them with least resistance. Take the example of cars. They have a streamlined shape to allow the air to flow around them easily with least resistance; without this streamlining, the air resistance would be much greater.

#### Question 1:

Discuss three situations in daily life where friction is an advantage?

Three situations in daily life where friction is an advantage:
1) Walking - Friction is required between the feet/footwear and the ground to prevents slips.
2) Mechanical Brakes - Mechanical brakes help in slowing down the motion of a machine whenever desired. For example, friction is required to bring a car to rest.
3) Matchbox - Friction between the match stick and the matchbox helps in generating heat; this heat helps in lighting the matchstick.

#### Question 2:

Explain with examples the disadvantages of friction.

Friction reduces the relative motion between two objects in contact. It is a disadvantage in many situations. Let us take the example of the mechanical engine used in cars. Friction between the mechanical components of the engine prevents smooth movements. This causes loss of energy in the form of heat, sound, etc. This reduces the efficiency of the car. If the engine is not properly lubricated, excessive friction may even damage its mechanical parts because of excess heat production. The excess sound coming out of the engine is also undesirable.

#### Question 3:

Explain four methods by which friction between two solid surfaces in contact can be reduced.

Following are the methods by which friction between two solid surfaces in contact can be reduced:
1) Lubrication - It helps in reducing roughness, which decreases friction.
2) Ball Bearings - Since rolling friction is less than sliding friction, ball bearings help in maintaining minimal surface of contact between two surfaces in motion.
3) Polishing - Smooth surfaces or bearings work better than the rough ones.
4) Streamline Design - A streamlined design prevents friction due to fluids.

#### Question 1:

If there was no friction, what would happen to a moving object?

Friction affects the relative motion between two surfaces in contact. Without friction, objects would not be able to move from a stationary position or stop when in motion. When a body is in motion and if there is no friction, it becomes impossible to bring it to rest. Think of a situation when you are travelling in a car and you cannot stop it because there is no friction between the surface of the tyres and that of the road. It will be a disadvantage in this case.

#### Question 2:

When you rub your hands together, they become warm. What is this due to?

Friction between the surfaces of hands produces heat, which makes them warm when rubbed together. When you rub your hands together, it causes friction. This leads to the production of heat, which makes your hands warm. This is because of the roughness of the surfaces of hands. It is a classic example of the conversion of mechanical energy to heat energy.

#### Question 3:

If you try to hold a glass with oil on your hands, it tends to slip. Why?

Friction between two surfaces in contact is reduced when the surfaces are lubricated. Oil is a lubricating agent. Hence, when you try holding a glass with oil on your hands, it tends to slip.

#### Question 4:

Why do you think rolling friction is less than sliding friction?

Rolling friction is less than sliding friction because of the minimal surface of contact between two surfaces. For a rolling body, the contact surface area is less and goes on changing. In case of rolling friction, we can assume that a point of a body is in contact with a surface; while in case of sliding friction, the whole surface area of the body is in contact with the surface.

#### Question 5:

Friction is a necessary evil. Explain.

Friction affects the relative motion between two bodies in contact. Without friction, objects cannot move from a stationary position or stop when in motion. Suppose you are walking on the road and encounter a smooth surface. You will fall because there will be no friction to hold your feet firmly on the surface. Now, let us change the situation. Let us suppose you are standing on a smooth surface and you are asked to walk on that surface. Again, you will not be able to walk properly because of lack of friction. Hence, friction is a necessary evil.

#### Question 1:

A businessman went to see a Formula 1 car race. It was raining and several cars skidded on the road. There were a couple of major accidents and some drivers sustained serious injuries.
He decided to do something about it. He designed a racing car tyre with increased friction that would not slip on wet roads. He advertised it as: "Now a racing car tyre that keeps you safe."
However nobody bought the tyres. Where do you think the businessman went wrong?

Due to increased friction, tyres of the car now will generate more heat and will burst out quickly. This would increase the rate of accidents even more. That's why nobody bought his tyres. For racing, car's tyres must have optimum frictional force, which neither produces much heat nor makes the motion too slippery.

#### Question 1:

Akash noticed that someone had carelessly thrown a banana peel on the footpath. He thought, "If someone steps on the banana peel, he or she is likely to slip and fall." So he picked it up and threw it in dustbin.
1. Is Akash right in thinking that people can slip on a banana peel? Why?
â€‹2. Which two values did Akash show?
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