Question

Why the total momentum before and after collision is equal?

Answer 1

Answer:

Because of the Newton 3rd law motion which states that "every action has a equal and opposite reaction".

Answer 2

The law of momentum conservation can be stated as follows. 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.