# Force, inertia and linear momentum

Force: The pull or push which change or tends to change the state of rest or of uniform motion or direction of motion of an object is called Force.

The units and dimension of force:

We know that                                  F = ma

There are two types of units of force

(i) Absolute Units:

(a) In M.K.S or S.I unit, the absolute unit of force is Newton (N)

Where                                                    1N = 1kg ´ 1m/s2

So 1N is the force which produces an acceleration of 1m/s2 in a body of mass 1 kg.

(b) In C.G.S system, the absolute unit of force is dyne

So 1 dyne is the force which produces an acceleration of 1 cm/s2 in a body of mass 1 gm.

(ii) Gravitational Units-:

(a) In M.K.S system or S. I unit, the gravitational unit of force is kg wt or kg F.

Where                                              1kg wt = 9.8 N

1kg wt is that much force which produces an acc. of 9.8 m/s2 in a body of mass1kg.

(b) In C.G.S system the gravitational unit of force is gm wt or gm F.

So 1 gm wt is that much force which produces an acceleration of 980 cm/s2 in a body of mass 1 gm.

Inertia: The inability of a body to change by itself its state of rest or of uniform motion or direction of motion unless some external force acts in it is called Inertia.

Linear momentum: The quantity of motion contained in a body is known as linear momentum. It is the product of mass of the body (m) and its velocity (v). It is denoted by p where

p = mv. It is a vector quantity.

The momentum has units kgm/sec.