The conservation law for momentum says that the total momentum of a closed system of objects—meaning one that isn't acted upon by any outside force—is constant. In practice, this means that no matter what happens inside the system, its initial momentum will always equal its final momentum.

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When abody of mass 'm, moving with a velocity u, hits on another body of same mass with a velocity v, then total momentum before collision is same as total momentum after collision. ie mv-mu=0