## INTRODUCTION

We use force all the time. We use force to open a door, to pick up the school bag, to brush our teeth, to squeeze out toothpaste from a tube, to turn on a tap and so on. Even the earth is exerting a force on us all time. It is pulling all of us and all things on earth towards it.In fact we use force for every single action of ours! You have studied about force in previous classes. Do you remember what a force is ?

FORCE

The external agent which tends to set a body in motion or which changes the speed and direction of motion of a body or which can change the shape of a body is called force.
(i) To push or to pull is also called as force.
(ii) It is a vector quantity

UNIT OF FORCE

(i) The SI unit (in standard international system) of force is called Newton and its symbol is N (kg × m/sec2)
(ii) C.G.S unit of force is dynes (gm cm/s2)
1 Newton = 105 dynes,
1 N     = kg – m/s2
= 1(1000 gm) (100 cm/s2)
= 105 gm–cm/s2
So that 1 N = 105 dyne

EFFECT OF FORCE

1. Force can move a body lying at rest
Example : (a) Kicking a stationary football.           (b)     Lifting a book kept on a table top.
(c) Hitting a stationary ball with a bat.

2. FORCE CAN STOP A MOVING BODY
Example :
(a) A goalkeeper stopping a football moving towards the goal post.
(b) A moving bicycle coming to rest on applying brakes. Here the frictional force between the brake lining of the brake shoe and the bicycle wheel rim, acts to stop the bicycle.

3.FORCE CAN CHANGE THE SPEED OF A MOVING BODY
Ex. To decrease speed, force is ap