Here's why:
* Conservation of Momentum: In a closed system (like the billiard table), the total momentum before the collision is equal to the total momentum after the collision.
* Momentum Transfer: When the balls collide, momentum is transferred from one ball to the other. One ball might lose momentum, while the other gains it.
* Net Change: Since momentum is transferred internally within the system, the overall change in momentum is zero.
Important Note: This assumes an ideal scenario where there are no external forces acting on the system (like friction from the table). In reality, there will be a tiny amount of momentum loss due to friction and other factors, but the change is usually negligible.
