Here's why:
* Closed system: A closed system is one that doesn't exchange matter or energy with its surroundings. This means no outside forces can act on the system.
* Momentum: Momentum is a measure of an object's mass in motion. It's calculated as mass times velocity.
* Conservation of momentum: A fundamental principle in physics states that the total momentum of a closed system remains constant. In other words, momentum can be transferred between objects within the system, but the total amount of momentum always stays the same.
Example:
Imagine a perfectly isolated box containing two billiard balls. If one ball collides with the other, momentum is transferred. The first ball loses momentum, and the second ball gains an equal amount of momentum. However, the total momentum within the box remains constant.
However:
If we introduce an external force, like someone hitting the box from the outside, this force would change the total momentum of the system. The box itself would gain momentum, and the momentum of the balls inside would also change.
Therefore, only external forces can change the total momentum of a closed system. Internal forces (like the collision between the billiard balls) only redistribute momentum within the system.
