RBSE Class 8 Science Important Questions Chapter 12 Friction

Rajasthan Board RBSE Class 8 Science Important Questions Chapter 12 Friction Questions and Answers.

RBSE Class 8 Science Chapter 12 Important Questions Friction

Objective Questions

Question 1.    
The cause of friction is the between the surfaces of contact.
(a) smoothness    
(b) regularity
(c) irregularity    
(d) (b) and (c) both
Answer:
(c) irregularity

  

Question 2.    
Lubricants     
(a) increase friction
(b) decrease friction
(c) are ineffective in friction
(d) (a) and (b) both
Answer:
(b) decrease friction

Question 3.    
Roller wheels     
(a) decrease the friction to zero
(b) increase the friction
(c) decrease the friction
(d) all of the above
Answer:
(c) decrease the friction

Fill in the blanks

1. .................... can be increased by making a surface rough.
Answer:
friction 

2. Friction can be decreased by using ....................
Answer:
lubricants

3. The frictional force due to .................... is also known as drag.
Answer:
fluids

4. .................... friction is less than sliding friction.
Answer:
rolling.

True/False

Read the following statements carefully and write 'T' for true and 'F' for false.
1. Friction reduces the relative motion between two surfaces in contact.
Answer:
True

2. Friction acts uniformly over all surfaces.
Answer:
False

3. Sliding friction is more than static friction.
Answer:
False

4. Friction can be increased by using lubricants.
Answer:
False

5. Friction can be increased by making the surfaces rough.
Answer:
True

6. Roller wheels decrease the friction.
Answer:
True

Match the words given m Column-A with 'Coiumn-B'

Question 1. 

Column-A	Column-B
(i) Dragging	(a) Comes into play when we try to move an object at rest
(ii) Rolling friction	(b) Substance used to reduce friction
(iii) Lubricants	(c) Friction due to liquid
(iv) Static friction	(d) Generated due to rolling objects

Answer:

Column-A	Column-B
(i) Dragging	(c) Friction due to liquid
(ii) Rolling friction	(d) Generated due to rolling objects
(iii) Lubricants	(b) Substance used to reduce friction
(iv) Static friction	(a) Comes into play when we try to move an object at rest

Very Short Answer Type Questions

Question 1. 
What do you understand by friction? 
Answer:
A force which opposes the relative motion between.

Question 2. 
Why are the tyres of cycles and cars treaded? Explain.
Answer:
The tyres of cycles and cars are treaded so as to increase friction between the tyres and the road. This friction gives better grip to the road.

Question 3. 
What is rolling friction?
Answer:
When a body rolls over any other surface the resistance to its motion is known as rolling friction.

Question 4. 
What is drag?
Answer:
The friction force exerted by fluids is called drag.

Question 5. 
Explain why it is easier to roll a cylindrical body than to pull that body by sliding?
Answer:
This is because we know that rolling friction is less than the sliding friction.

Question 6. 
What are lubricants?
Answer:
Liquids which are used to reduce friction are called lubricants.

Question 7. 
What is a spring balance?
Answer:
Spring balance is a device which measures the force acting on a body.

Question 8. 
What do you mean by static frictional force? 
Answer:
The frictional force which acts on a body in the state of rest so as to balance the external forces acting on the body is called static frictional force.

Question 9. 
What do you mean by sliding frictional force?
Answer:
When a body slides over another body then the opposing force acting between the surfaces of contact is known as sliding frictional force.

Question 10. 
It is easier to push a moving box than to move a stationary box from rest to motion. Why?
Answer:
We know that the value of sliding friction is less than static friction. Hence this is the reason that it is easier to push a moving box than to move a stationary box from rest to motion.

Short Answer Type Questions

Question 1. 
How is the magnitude of force measured using a spring balance?
Answer:
Spring balance is a device which can measure the force acting on a body. It consists of a helical spring which expands on the application of force and extension in the spring is proportional to the force applied. Initially the spring is calibrated by applying known forces and marking the extensions. Later on when some unknown force is applied to the spring balance, then knowing the extension we can measure the unknown force.

Question 2. 
What is the cause of friction?
Answer:
Friction between two surfaces is caused by the irregularities of two surfaces in contact. The surfaces which appear to be very smooth also have a large number of microscopic irregularities. The irregularity of one surface is interlocked with the irregularity of the other surface. When we try to slide one such surface over a similar surface, we have to apply some force to overcome the interlocking. This force appears as a frictional force.

Question 3. 
"If two forces are pressed using forces then the friction will increase." Explain giving an example.
Answer:
Pull a mat when nobody is sitting over it. Again pull the mat when some body is sitting over it. We will experience that we had to apply more force to pull the mat when some body was sitting over the mat. It explains that frictional forces increase if the surfaces are pressed using forces.

Question 4. 
"Friction can generate heat", explain giving examples.
Answer:

1. If we vigorously rub our palms together for few seconds, we will feel that heat is generated.
2. When a match stick is rubbed against a rough surface, we find that it catches fire.
3. When the kitchen mixer is used for few minutes, it generates heat.

Question 5. 
Explain the following:
1. Static friction 
2. Limiting force of friction 
Answer:
1. Static friction:
If the body does not move even after applying a force, it means that a similar and opposite force is produced on it. In this condition, the opposite force is called static friction. As the magnitude of applied force increases, so does the static frictional force. 

2. Limiting force of friction:
If we keep on increasing the applied force, the friction force increases to a limited value. This limited friction force value is called limiting friction. In this condition, the object is about to slide on the horizontal surface.

Question 6. 
Explain sliding friction.
Answer:
When applied force is more than the limiting force of friction, the body starts moving. In this case, the effective force on the object is equal to the difference between applied force and limiting force of friction. Similarly, when one object slides over the other, friction produced is called sliding friction. The value of sliding friction is slightly less than limiting friction. 

Question 7. 
How do we see writings on the blackboard?
Answer:
When we write on the blackboard, the rough surface bf blackboard, due to rubbing removes some particles from the chalk which stick to the blackboard and we see the writings.

Question 8. 
Why are lubricators used in machines? 
Answer:
A lubricant spreads as a thin layer between two surfaces. Now the movement takes place between the surface and lubricant and friction changes from air friction to fluid friction. Since fluid friction is less than air friction, therefore to reduce friction, between different parts of the machine, lubricant is used.

Questsion 9. 
Why is it difficult to run on smooth surface?
Answer:
Friction force between shoes and smooth surface is excessively low. As we run on smooth surface, we are unable to press the surface appropriately, and this way, we are not able to obtain an appropriate reaction to move forward.

Question 10. 
What is friction? Write one method each to increase or reduce friction.
Answer:
Friction:
Force produced due to irregularities of two surfaces in contact is called friction. Surfaces which appear to be smooth, also have a large number of microscopic irregularities. These irregularities on the two surfaces lock into one another. When we try to move any one surface over the other, we have to apply a force to overcome interlocking.

This is the cause of friction. Oil, grease and graphite are used to reduce friction. They are called lubricants. They, when applied between two surfaces form a thin layer between them. It reduces friction. In order to increase friction, tyres of cars, trucks and other vehicles are treaded. This improves their movement on road.

Question 11. 
How do wheels reduce friction?
Or
What is rolling friction? How does it reduce friction?
Answer:
When an object rolls over the surface of the other object, the resistance to its motion is called rolling friction. Rolling reduces friction. Rolling of an object over another is easier than sliding of one object over other. This is the reason why wheeled objects are easy to pull. Because rolling friction is less than sliding friction, therefore most machines have sliding friction transformed to rolling friction by use of ball bearings. Therefore, its clear that objects with roller wheels reduce friction.

Question 12. 
What do you mean by fluid friction? 
Answer:
We know that air and liquid resist the motion of objects, that is, they apply friction force. In science, gas and liquids are considered as fluids, therefore fluids apply friction on objects moving through it. It is also called Drag.

Essay Type Questions

Question 1. 
Write the methods to reduce friction. 
Answer:
Friction can be reduced by following methods:
1. By polishing the surface:
For reducing the friction, the surfaces are polished. The irregular surface is filled with the polish and therefore, the friction is reduced.

2. Use of a lubricant:
For reducing friction, lubricants like oil or grease are used. When oil or grease is applied between two surfaces, surfaces and their irregularities remain separated and therefore interlocking doesnot take place. By this, surfaces can have relative motion between them with reduced friction.

3. By making a streamline body:
When vehicles travel at high speeds, they have to suffer huge air friction. To reduce the effect of air friction on the speed of vehicle, the front part of the body is made streamline, e.g. the front of aeroplane looks like fish.

4. Sliding friction is converted into rolling friction:
Rolling friction is less than sliding friction. This is why sliding friction is turned into rolling friction. This is done by using ball bearing.

Question 2. 
Suppose there is no friction. How will the life be affected? Write a list of ten such possibilities to be affected.
Answer:
If friction is suddenly removed, the ten situations that will be thus produced are following:

1. We will not be able to walk on earth surface.
2. We will not be able to write.
3. On applying brakes to moving vehicles, they will slip.
4. We cannot write on blackboard.
5. Rotation in various parts of machine won't be possible.    
6. Nails and screws won't be able to hold tightly two objects together.
7. We won't be able to light a match stick.
8. Vehicle will not be put into motion.
9. We will not be able to tie a knot in a thread.
10. Buildings cannot be built.