# Friction Class 8 Extra Questions and Answers Science Chapter 12

In this page, we are providing Friction Class 8 Extra Questions and Answers Science Chapter 12 pdf download. NCERT Extra Questions for Class 8 Science Chapter 12 Friction will help to score more marks in your CBSE Board Exams.

## Class 8 Science Chapter 12 Extra Questions and Answers Friction

Extra Questions for Class 8 Science Chapter 12 Friction with Answers Solutions

### Friction Class 8 Extra Questions Very Short Answer Type

Question 1.
What is the force of friction?
The force acting on a moving object equal and opposite to the direction of motion is called force of friction.

Question 2.
What is the cause of friction?
The irregularities on the two surfaces in contact is the cause of friction.

Question 3.
What is the force responsible for wearing out of bicycles tyres?
Frictional force

Question 4.
Is the friction same for all objects?
No

Question 5.
Why are wheels of vehicles made circular?
Rolling friction is less than sliding friction.

Question 6.
What is the direction of force of friction acting on a moving object?
Opposite to the direction of moving objects.

Question 7.
Why do pieces of luggage fit with rollers?
To reduce friction and to pull them easily.

Question 8.
Why does a matchstick catch fire when rubbed on the rough surface of the box?
Due to friction between the stick and rough surface a matchstick catches fire.

Question 9.
What is the name of special shape given to objects moving in fluids?
Streamlined

Question 10.
Give one example where ball bearings are used.
Ceiling fan

Question 11.
Does friction depend on the nature of objects?
Yes

Question 12.
What is the use of lubricants?
They are used to reduce friction.

Question 13.
Which surface does produce more friction smooth or rough surface?
Rough surface

Question 14.
Why is fine powder sprinkled on the carrom board?
In order to reduce friction powder is sprinkled on carrom board.

Question 15.
Why is it difficult to walk on the wet marble floor?
The wet marble floor has less friction. So it is difficult to move on it.

Question 16.
Why does the jar of a mixer become hot when it is run for few minutes?
It becomes hot due to friction.

Question 17.
Name the types of friction.

• Static friction
• Sliding friction
• Rolling friction
• Fluid friction

Question 18.
Which force helps us to write on the paper or blackboard?
Frictional force

Question 19.
What is drag?
The frictional force exerted by fluids is called drag.

Question 20.
What are fluids?
Commonly gases and liquids are called fluids.

Question 21.
Give some examples having streamlined body.
Aeroplanes, birds, ships, etc.

Question 22.
Oil, creams and grease are called lubricants. Why?
Because they reduce friction.

Question 23.
How can fluid friction be minimised?
By giving suitable shapes to bodies moving in fluids.

Question 24.
When does static friction come into play?
When we try to move an object at rest, static friction comes into play.

Question 25.
Can we reduce friction to zero?
No

### Friction Class 8 Extra Questions Short Answer Type

Question 1.
If we push the book on the table, it stops after sometime. Why?
When we push the book on a table, a force is applied on it in the opposite direction of the motion called force of friction. This force opposes the movement and the book stops.

Question 2.
Why are the worn-out tyres discarded?
The worn out tyres are discarded because their tread are worn off and limiting their effectiveness of providing required amount of friction.

Question 3.
Why is it easier for the fish to swim in water?
Fish can easily swim in water because it has a streamlined body which produces less friction. So it easily cuts through water and moves in it very easily.

Question 4.
Give some examples to show that friction produces heat.
The examples are:

• Rubbing of our hands to keep us warm in winter.
• Rubbing of matchsticks on the rough surface to make fire.

Question 5.
Give some examples of smooth and rough surfaces.
Smooth surfaces: Surface of the mirror, surface of wet soaps, glazed tiles, surface of an oily pot, etc.
Rough surfaces: Surface of plyboard, unpainted wall, brick, tyres, etc.

Question 6.
What is a spring balance?
A spring balance is a device which measures the force acting on an object. It contains a coiled spring. When a force is applied on it, the spring gets stretched. The pointer moves on the scale showing the magnitude of the force.

Question 7.
How does the friction depend on the nature of the surface?
Nature of the surface is one of the factors affecting friction. When we move on any surface, we apply a force to overcome the interlocking of the surfaces. Rough surfaces have a larger number of irregularities and greater frictional force.

Question 8.
What are the two factors on which the force of friction depends?
These two factors are:

• The nature, i.e., smoothness or roughness of the surfaces in contact.
• How the surfaces in contact pressed together.

Question 9.
When the two surfaces are pressed harder, friction increases. Explain why?
The friction is caused by the interlocking of irregularities in the two surfaces. It is obvious that the force of friction will increase if two surfaces are pressed harder. This can be experienced by dragging a mat when nobody is sitting on it and when a person is sitting on it.

Question 10.
What is a measure of static friction?
The force required to overcome friction at an instant an object starts moving from rest is a measure of static friction.

Question 11.
What is a measure of sliding friction?
The force required to keep the object moving with the same speed is a measure of sliding friction.

Question 12.
Why is it easier to move the box already in motion than to get it started?
or
The sliding friction is slightly smaller than the static friction. Explain why?
When the box 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. Hence, it is somewhat easier to move the box already in motion than to get it started.

Question 13.
Why are the soles of shoes and tyres of vehicles grooved?
The soles of shoes and tyres of vehicles like cars, trucks, etc., are grooved to provide better grip on the floor or roads to move safely because grooving increases friction.

Question 14.
We use ball bearings between the hub and axle of ceiling fan and bicycles. Why?
The rolling friction is smaller than the sliding friction. In most machines, sliding is replaced by rolling by using ball bearings to reduce friction. This is why ball bearings are used between the hub and axle of ceiling fan and bicycle.

Question 15.
Why are lubricants applied between the moving part of a machine?
When lubricants are applied between the moving part 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 and the movement becomes smooth.

Question 16.
How brake stop moving bicycles and automobiles?
When we press the brake lever, the brake pads arrest the motion of the rim due to friction and the wheel stops moving. This is how brake stops moving bicycles and automobiles.

Question 17.
People often rub their hands in winter. Why?
Friction produces heat. When we rub our hands together, the friction arises between them and heat is produced. This makes our hands warm. This is why people often rub their hands in winter.

Question 18.
Which factors affect fluid friction?
The speed of the object with respect to the fluid, shape of the object and nature of the fluid affect fluid friction.

Question 19.
In some cases, we deliberately increase friction. Explain why?
We need to increase friction in some cases like grooving in the sole of shoes, the treaded tyres of cars, trucks or other vehicles, using brake pads in the brake system of bicycles and automobiles. If we do not do so, we may slip on the floor or roads, vehicles may slip on the roads and they will not stop, our bicycles and automobiles will not stop, etc.

### Friction Class 8 Extra Questions Long Answer Type

Question 1.
What are the various types of friction? Explain.
There are mainly three types of friction:
(i) Static friction: The friction existing between a stationary object and the surface on which it’s resting is called static friction. This type of friction is more powerful than others. We need a lot of energy to overcome such type of friction.

(ii) Sliding friction: The friction between the two surfaces in contact when one of them just slides , over the other is called sliding friction. It is also known as kinetic friction. It is less than static friction.

(iii) Rolling friction: When one body rolls over the surface of another body, the resistance of its motion is called rolling friction. Rolling friction is less than sliding friction. It is thus the least friction of three types of friction, i.e., Static friction > Sliding friction > Rolling friction.

Question 2.
What are the causes of friction? Explain in detail.
Friction is mainly caused by the following factors:
(i) The interlocking of surfaces: The force of friction arises due to interlocking of the irregular projections on the two surfaces. All surfaces have many irregularities. When we attempt to move one object over the other, we have to apply a force to overcome interlocking of the irregularities in their surfaces and friction is produced. The roughness varies from surface to surface due to varying irregularities on the surfaces. So the more is the roughness of the surface, the larger is the number of irregularities and hence the greater will be the friction.

(ii) Force of adhesion: When the two rough surfaces come in contact, it leads to the formation of various contact points. The atoms or molecules present at such points of contact give rise to attractive forces of adhesion that opposes the relative motion of the bodies. Hence, the force of adhesion acting between two surfaces cause friction.

(iii) Deformations: Soft materials will deform when under pressure. This also increase the resistance of motion. For example, when you stand on a sand bed, you sink slightly, which causes resistance when you try to drag your feet along the sand surface. In this case, you must plow through to move, thus creating resistive force.

(iv) Weight of the body: The greater is the weight of the body, the greater will be the force of friction. If we push a heavy box, it will require greater force to move as the force of friction is greater. On the other hand, it is easier to move a lighter box because it produces less friction.

Question 3.
Explain increasing and decreasing friction with suitable examples.
In our daily life, in many cases we increase or decrease friction for our own convenience. Some examples are given below:
Increasing friction:

• Soles of our shoes are grooved to provide the shoes better grip on the floor so that we can move safely. Similarly tyres of vehicles are treaded to increase friction to provide better grip.
• We increase friction by using brake pads in the brake system of bicycles and automobiles.
• Gymnasts apply some coarse substance on their hands to increase friction for better grip.
• Kabaddi players rub their hands with soil for a better grip of their opponents.
• Handle of cricket and tennis are made of rough materials to get better grip.

Decreasing friction:
In some situations, however, friction is undesirable and we would want to minimise it.
For example:

• A few drops of oil are poured on the hinges of a door, the door moves smoothly.
• A bicycle and motor mechanic uses grease between the parts of these machines to reduce friction.
• By polishing surfaces, sprinkling powder on carrom board, we reduce friction.
• Wheel and ball bearings are used in machines and vehicles to reduce friction.

Question 4.
What do you mean by fluid friction? How can fluid friction be reduced?
The gases and liquids are called fluid. All of them exert force of friction on solid bodies moving in either of them. The force exerted by fluid is called drag. It depends on speed of the object with respect to fluid. It also depends on the shape of the object and the nature of the fluid. is obvious that when objects move through fluids, they have to overcome friction acting on them.

In process they lose energy Efforts are, therefore, made to reduce friction. So, the objects are given special shapes Birds and fishes have to move about in fluide all the time. Their bodies must have evolved to shapes which would make them lose less energy in overcoming friction. Such shapes are called streamlined This is why shapes of aeroplanes, ships and all vehicles are designed to have shapes which minimise fluid friction.

Question 5.
Some points are given below which show that friction is both a friend and a foe:
(a) Friction as friend:

• It allows us to grip and catch any object.
• It helps us to walk comfortably on the floor.
• It helps to minimise the speed or to stop any moving object.
• It helps us to write.

(b) Friction as foe:

• It causes wear and tear in objects.
• It causes damage to the parts of machines and tools which further require money to get them
repaired.
• It reduces the speed of moving objects, so more force is required.
• It produces hurdles in moving any object freely.

### Friction Class 8 Extra Questions HOTS

Question 1.
Will force of friction come into play when a rain drop rolls down a glass window pane?
Yes

Question 2.
Will an object experience any force of friction which is falling through a long straight glass having no air in it?
No

Question 3.
Cartilage is present in joints of our body, which helps in their smooth movement. 1f cartilage wears off, how would this affect the movement of joints?
Wearing off cartilage will make its surface rough and increase the friction. As a result the movement of joints will become difficult which may lead to joint pains.

Question 4.
Why handle of cricket bat or badminton racquet is rough?
To have a better grip by increasing friction between handle of the bat and hands.

Question 5.
Two identical metal sheets, A and B, are rubbed with paper and sand paper respectively. Which one of them will shine more?
Metal sheet B will shine more because friction between sandpaper and metal sheet is larger than friction between paper and metal sheet. Thus, sandpaper will be able to remove the outer dull layer of metal more effectively than other.

### Friction Class 8 Extra Questions Value Based (VBQs)

Question 1.
On Sunday Razak invited everybody to play carrom in his house. While playing, striker was not moving properly from one place to another. Everybody thought it is their bad luck. But Razia. one of the player. quickly got up and sprinkled some talcum powder on the hoard.

(a) Why Razia sprinkled some talcum powder?
(b) Can you give sumo other example which serves the same purpose!
(c)What value of’ Razia is shown here?
(a) Reala sprinkled some talcum powder to make the surface of carrom board smooth and hence reduce friction.
(b) Friction can be reduced by using lubricants, hail bearings and painting the rough surface.
(c) Razia is an intelligent girl with scientific aptitude.

Question 2.
Rani and Shyam while returning from school saw labour pushing hard to move a heavy box B. immediately ran to him and started helping hint in pushing the box whereas Shyam brought two in bars and asked the labour to put them below the box and then to push the box.

(a) I What are the factors that affect friction?
(b) I Who will help the labour more Ram or Shyaxn? How?
(c) What values of Raw and Shyam are shown here?