Force:
It is
defined as an external agent, which tends to produce the motion to a body.
According to
Newton’s law of motion, force is directly proportional to rate of change of
momentum.
F=ma
Explanation:
We know that
Momentum= mass*velocity (ie., change
in velocity)
m =mass of abody
u = initial velocity
v = final velocity
a = acceleration
t = time taken to change
the velocity from u to v.
Momentum = mass(final
velocity- initial velocity)
= m(v-u)
Rate of
change in momentum= 
Rate of
change in momentum = ma
As force=
momentum
F=ma