In a closed system, the total momentum before collision is equal to the total momentum after collision. This is a consequence of Newton's third law of motion, which states that for every action, there is an equal and opposite reaction.

In other words, when two objects collide, they exert forces on each other that are equal in magnitude but opposite in direction. This means that the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision.

This can be seen mathematically by considering the law of conservation of momentum:

```

p = mv

```

where:

* p is momentum

* m is mass

* v is velocity

The law of conservation of momentum states that the total momentum of a closed system remains constant. This means that the total momentum before the collision is equal to the total momentum after the collision:

```

p_before = p_after

```

where:

* p_before is the total momentum before the collision

* p_after is the total momentum after the collision

This equation can be used to solve for the velocity of either object after the collision, given the velocities of both objects before the collision and the masses of both objects.