E. Very short answer type questions : 1. Define the force of friction. 2. What type of force is friction? 3. What is a lubricant? 4. Define rolling friction. 5. Define sliding friction. 6. Name two machines which use streamlining to reduce friction. F. Short answer type questions : 1. How friction is useful to us. Briefly explain with two examples. 2. Name various types of friction. 3. Differentiate between rolling and sliding friction. Also give two examples 4. What are the factors that affect friction. Explain. 5. Explain briefly effects of friction. 6. Explain few methods to increase friction. Also give two examples. 7. Give three examples where friction is advantageous and three examples where friction is disadvantageous to us. G. Long answer type questions : 1. Explain all types of friction with example. 2. Friction a necessary evil. Justify? 3. Why it's necessary to decrease friction? 4. Describe the ways to decrease friction. 5. Why it's necessary sometime to increase friction?​

E.

1. The force that opposes the relative motion between two surfaces of objects when they come in contact.

2. Friction is the force that resists motion when the surface of one object comes in contact with the surface of another.

3. A substance that is introduced between two surfaces in contact, to reduce friction, is called a lubricant.

4. It is the force resisting the motion when a body (such as a ball, tire, or wheel) rolls on a surface.

5. Sliding friction is a frictional force that appears when two bodies come into sliding contact.

6. A few machines which use streamlined shape to reduce friction are - Aeroplanes, Ship, Boat, Rockets, Missiles, Cars.

F.

1. Friction can be a useful force because it prevents our shoes slipping on the pavement when we walk and stops car tyres skidding on the road.

For example, we use oil to reduce the friction between the moving parts inside a car engine. The oil holds the surfaces apart and can flow between them.

2. There are four types of friction: static, sliding, rolling, and fluid friction.

3. Rolling friction: Rolling friction takes place when an object rolls on the surface.

Rolling friction takes place due to the deformation of surfaces.

Ex : A bike with a broad tire will burn more fuel because of the increased rolling friction.

Sliding friction: Sliding friction takes place when two surfaces are rubbed against each other.

Sliding friction takes place due to interlocking between microscopic surfaces.

Ex : Rubbing both the hands together to create heat.

A child sliding down through a slide in a park.

4.

•Surface Roughness.

•The weight on the surface.

•The texture of the surface.

•The angle of the surface.

•Adhesion.

5. Friction prevents vehicles from slipping on the road. The disadvantages of friction are that it may slow down or stop the motion of objects, and also cause the generation of heat. Such heat can lead to energy wastage in machines. It can also cause wear and tear.

6. Friction can be increased by increasing the roughness of the surfaces in contact. For example, treading of shoes and tires is done to increase friction. When friction is undesirable we have to reduce the friction.

7. Advantages:

•Friction helps us to walk turn and stop

•Friction helps us to wear caps and ring

•Friction helps us in the transfer of energy

•Friction helps us in making fire

•Friction helps us to hold on to objects

Disadvantages:

•Produces unwanted heat

•Reduces efficiency of machines

•Eco-Hazard

•Produces unwanted noise

•Causes wear and tear

G.

1.

a) Static Friction

Static friction is defined as the frictional force that acts between the surfaces when they are at rest with respect to each other.

Examples:

•Skiing against the snow

•Creating heat by rubbing both the hands together

•Table lamp resting on the table

b) Sliding friction

Sliding friction is defined as the resistance that is created between any two objects when they are sliding against each other.

Examples:

•Sliding of the block across the floor

•Two cards sliding against each other in a deck

c) Rolling friction

Rolling friction is defined as the force which resists the motion of a ball or wheel and is the weakest types of friction.

Examples:

•Rolling of the log on the ground

•Wheels of the moving vehicles

d) Fluid friction

Fluid friction is defined as the friction that exists between the layers of the fluid when they are moving relative to each other.

Examples:

•The flow of ink in pens

•Swimming

2. Friction is said to be a necessary evil because it is useful as well as harmful. Friction helps us to walk, write, hold things, lift objects. So it is necessary for our lives. Without friction, many essential processes can not be done.

3. Friction is a necessary evil which causes a lot of wear and tear in machine parts that move against each other. It erodes the surfaces and destroys their symmetries. Hence, in such cases it is desirable to reduce friction. Friction is reduced by lubricating surfaces to decrease the friction.

4. Friction can be reduced by:

•lubricating the surfaces.

•use of ball bearings (i.e. replacing sliding friction with rolling friction).

•streamlining the body.

5. Sometimes friction between two surfaces decreases to such an extent that it is difficult to move on that surface. So, friction needs to be increased. For example, vehicles can't move on a road covered with snow. In such cases we have to throw sand on the road to increase the friction.