Newton law of motion:
The rate of change of momentum is proportional to external force applied on the body and take place, in the same direction in which the force acts.
Let a body having mass 'm' is moving with a velocity 'u' along a straight line. It is acted upon some force 'F'
and the velocity of 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
That is, Rate of change of momentum = m.a
But according to second law F proportional to m.a
That is F = k.m.a Where K = constant.
Unit of force
1N = 1 kg-m/sec2 = 105 dyne = 1 grm.cm/sec2