Tia a stone to one end of a string. Take the other end in your hand and rotate the string so that the stone moves along a circle as shown in figure 1.1 (a). Are you applying any force on the stone? In which direction is this force acting ? How will you stop this force from acting ? What will be the effect on the stone ?

Explanation

As long as we are holding the string, we are pulling the stone towards us, i.e., towards the centre of the circle and are applying a force towards it. The force stops acting if we release the string. In this case, the stone will fly off along a straight line which is the tangent to the circle at the position of the stone when the string is released, because that is the direction of its velocity at that instant of time. Thus, a force acts on any object moving along a circle and it is directed towards the centre of the circle. This is called the centripetal force. 'Centripetal' means centre seeking, i.e., the object tries to go towards the centre of the circle because of this force.