* Momentum is conserved in a closed system: The total momentum of a system *before* a collision is equal to the total momentum of the system *after* the collision.

* Individual momentum changes: While the total momentum remains constant, the individual momentum of each object involved in the collision will likely change. This is because momentum is a vector quantity, meaning it has both magnitude and direction.

Here's a breakdown:

* Before the collision: Each object has its own momentum (mass x velocity).

* During the collision: Forces act between the objects, causing their velocities to change.

* After the collision: The objects will have new velocities, and therefore new momenta. However, the vector sum of their momenta will be equal to the vector sum of their momenta before the collision.

Example:

Imagine two billiard balls colliding head-on. If the first ball is moving right and the second ball is stationary, the first ball will slow down and the second ball will start moving to the right. Their individual momenta have changed, but the total momentum of the system (both balls) remains the same.

Key takeaway: While the total momentum remains constant, the momentum of individual objects changes during a collision.