It states that the total momentum of any system of objects remains constant in the absence of any external force. According to the law of conservation of momentum, for a collision between two bodies
Total momentum before collision = Total momentum after the collision.
m1u1 + m2u2 = m1v1 + m2v2
Example: When a shot is fired from a gun, the gun recoils. When the shot is fired, it leaves the barrel with a certain force. The outgoing shot causes a reaction on the gun. Thus an equal and opposite force acts on the gun and it recoils.
