Answer:
To explain conservation
of momentum, let us take the following example. Consider two balls A and B
having masses 
and 
respectively. Let the initial
velocity of ball A be 
and that of ball B be
. Their collision takes place
for a very short interval of time t and after that A and B start moving with
velocities 
and 
(now
) respectively as shown in
Figure.
The momentum of ball
A before and after the collision is 
and respectively. If there are no
external forces acting on the body, then the rate of change of momentum of ball
A, during the collision will be 
and,
similarly the rate of change in momentum of ball B 
Let 
be the force exerted by ball A
on B and be the force exerted by ball B
on A. Then, according to Newton's second law of motion
and 
According to
Newton's third law of motion, we have
Or, 
Or, 
i.e., Total momentum before
collision = Total momentum after collision Thus, we find that in a collision
between the two balls the total momentum before and after the collision remains
unchanged or conserved provided no net force acts on the system. This result is
law of conservation of momentum.
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