Momentum, the product of the mass of a particle and its velocity. For a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2.
Therefore, according to the law of conservation of momentum, the total momentum is equal before the after the collision. So the total momentum remains the same.