Force

Simply, pull or push is called force. A book lying on the table continues to lie on the table unless an external force is applied on it. So, generally force is an external agency which changes or tends to change the state of a body from rest to motion or vice versa. The SI unit of force is newton (N). In CGS system, the unit of the force is dyne. 1 newton= 10⁵ dynes.

Consider a situation in which football is rolling in the ground. Ball stops rolling after some time. If a ground is perfectly smooth then ball will continue rolling forever in same direction with constant speed. It is a roughness of the surface that provides opposing force, which is called friction, to slow down the speed of the ball and finally stop the ball. Friction can be defined as the opposing force which acts on the opposite direction of motion.

Force acting on a body can do following things:

• It can change the speed of the body.
• It can change the direction of motion of the body.
• It can change the shape of the body.
• It can change the position of the body.

Balanced and unbalanced forces

When a number of forces acting on a body do not change its states of rest or uniform motion in a straight line, the force are said to be balanced forces. For example, in the game tug of war, a game where two teams pull a rope in opposite direction. In this game rope remains in steady and does not move in any direction when two teams exert equal and opposite force in a rope.

When a number of forces acting on a body change its state of rest or uniform motion in a straight line, the forces is said to be unbalanced forces.

For example, if we push a toy car towards east, it moves towards east. This unbalanced force produces motion.