**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/s^{2}

So 1N is the force which produces an acceleration of 1m/s^{2} 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/s^{2} 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/s^{2} 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/s^{2} 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.