The entire system of Dynamics is based on three laws of motion, which are the basis assumptions, and were formulated by Newton.

**First Law **

A particle remains at rest (if originally at rest) or continues to move in a straight line (If originally in motion) with a constant speed. If the resultant force acting on it is Zero.

It is also called the law of inertia, and consists of the following two parts:

A body at rest has a tendency to remain at rest. It is called inertia of rest.

A body in motion has a tendency to preserve its motion. It is called inertia of motion.

**Second Law **

The rate of change of momentum is directly proportional to the external force applied on the body and take place, in the same direction in which the force acts.

Let a body of mass ‘m’ is moving with a velocity ‘u’ along a straight line. It is acted upon a force ‘F’ and the velocity of the body becomes ‘v’ in time ‘t’ then.

Initial momentum = m.u

Initial momentum = m.v

Change in momentum = m(v-u)

Rate of change of momentum = change of momentum / Time

= m(v-u)/t but

v = u + a.t a = (v-u)/t

i.e Rate of change of momentum = m.a

But according to second law F proportional to m.a i.e.

F = k.m.a Where K = constant.

Unit of force

1N = 1 kg-m/sec^{2} = 10^{5} dyne = 1 grm.cm/sec^{2}

**Third Law **

The force of action and reaction between interacting bodies are equal in magnitude, opposite in direction and have the same line of action.