What does the law of conservation of momentum states?

Newton's Third Law of Motion states that, if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

Assume a closed system in classical mechanics or an isolated system in thermodynamics. This is a system which does not exchange any matter outside and is not acted upon by any outside forces. In these theoretical conditions a mass m1 traveling at velocity u1 hits a mass m2 traveling at velocity u2 their velocities change to v1 and v2:

m1u1 + m2u2 = m1v1 + m2v2

So, simply put: In the absence of outside forces, the total momentum of two colliding objects does not change as a result of the collision of the objects. However, there are no perfect inelastic collisions. There are energy losses, mainly due to the generation of heat in the colliding bodies and the release of sound energy into the surroundings. This is the reason that a Newton's cradle eventually stops.