Here's why:
* Momentum: Momentum is a measure of an object's mass in motion. It's calculated as mass * velocity.
* Conservation of Momentum: This law states that the total momentum of a closed system (one where no external forces act) remains constant. In other words, in a collision, the total momentum of the objects *before* the collision is equal to the total momentum of the objects *after* the collision.
Example:
Imagine two billiard balls colliding. The total momentum of the two balls before they hit is equal to the sum of their individual momenta. After the collision, even though the individual velocities of the balls may change, the total momentum of the system (the two balls) remains the same.
Important Notes:
* This law applies to all collisions, whether they are elastic (energy is conserved) or inelastic (energy is lost, like in a car crash).
* External forces, like friction, can change the momentum of a system.
