Momentum – the sum of mass multiplied by velocity for each particle – is conserved in collisions as a consequence of Newton's third Law, which states:
To every action, there is an equal and opposite reaction.
This implies also that the change in momentum for each particle, called the impulse, expressed as 
and also written as Force*time, is equal and opposite for any two particles in a collision.
Using the principle of conservation of momentum, 
Example
A bullet with a mass of 0.02 kg leaves a gun at 1500 m/s. If the gun’s mass is 1.2 kg, what is the velocity of the recoil on the gun?
What is the impulse exerted on a)the gun? b)the bullet?
c)If the explosion lasts 0.01 seconds find the acceleration of c)the gun? d)the bullet?
Initially the total momentum is zero since neither gun or bullet are moving, so
a)
b)
c)
d)
