# 4th Class Science Work, Force and Energy Types of Energy

## Types of Energy

Category : 4th Class

### Types of Force

In this chapter, we will study about two types of forces:

(i) Frictional force

(ii) Gravitational force

Friction

Friction is a type of force exerted when two surfaces are in contact with each other. Lets us understand it in a simpler way with an example.

A bicycle gradually slows down and finally stops, when we stop pedaling. A ball that you have rolled, gradually slows down and then comes to rest. When you push your toy, it moves for some distance and then slow down and finally stops after some time. These are few of the events that most of you have experienced. Can you guess why the moving object comes to rest automatically?

In all the above cases there is no force that appears to act on the object, then why the object's speed slows down and finally stops? Or, is there some force acting on these objects? Yes, the force is known as friction.

Friction always acts on all the moving objects, but the direction of force of friction is always opposite to the direction of motion of the object. In all the above cases, it is the force of friction which slows down the speed and finally stops the moving objects, like moving a ball, bicycle or a moving toy. Surfaces in contact with each other, exert force on each other. This force exerted by the surface is in the opposite direction to the other. This causes resistance of motion of one surface on the other. This opposing force is called the force of friction or even as force of resistance. This opposing force is the main cause of slowing down of the movement of the object and its complete stop.

We need this force of friction in all our actions. If there was no friction we would not be able to walk, run, play or doing any such activities. Anything that started to move would never stop, a toy would keep moving or the ball would keep rolling. Imagine how the world would be with object once set in motion always remaining in the same condition.

Force of friction depends on the surface of the moving object. In general, rough surfaces like, stoned or kacha road, floor with carpet has more friction as compared to smooth floor

Gravity

Gravity is the force with which objects are pulled downwards towards the centre of the earth. Let us understand it better with an example.

Body released                 Forces on body                          Forces on body at

from rest                        acceleration                               terminal velocity

Ball falling down due to the force of gravity

You must have observed that when you throw a ball up, it acquires certain height and then comes back toward the surface of the earth. Have you ever given it a thought why this happens? Well, this happens because of the force of gravity. Actually what happens is, when you toss the ball up in the air, you apply a force on the ball. This moves the ball up against the gravitational force applied by the earth. But due to the force of gravity, speed of the ball gradually slows down and finally stops for a while. And then it returns toward the centre of the earth. Thus we can say that force of gravity is an attractive force that pulls all the objects towards the centre of the earth. The force of gravity acts on all of us and on all object, all the time, without our being aware of it. It was Sir Isaac Newton (1642-172) who discovered this force of gravity.

Fruits or leaves of the plants fall towards the ground, the water comes down wards from the tap, a pen or a note book falls down wards only, are the examples showing the force of gravity working on them.

When we push an object it covers some distances and finally gets stopped. Which one of the following forces causes this?

(a) Gravity

(b) Friction

(c) Muscular force

(d) Nuclear force

(e) None of the above

Explanation

Friction is the resisting force which stops the movement of moving object

Force exerted by all matter on all other matter. Name the force.

(a) Magnetic force

(b) Frictional force

(c) Electrostatic force

(d) Gravitational force

(e) All of the above