The entire system of Dynamics is based on three laws of motion, which are the basis assumptions, and were formulated by Newton.
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.
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/sec2 = 105 dyne = 1 grm.cm/sec2
The force of action and reaction between interacting bodies are equal in magnitude, opposite in direction and have the same line of action.