線動量(linear momentum)

解釋桌球的線動量不守恆。
The linear momentum ( 線動量) of the billard ball was not conserved because there was external force acting on the ball when it collided with the side pad of the table.
If you resolve the original linear momentum into two directions, one is parallel to the side of the table and the other perpendicular to it. The momentum that is parallel to the side of the table would not be affected by the collision, but there is a change of momentum for the one peprpendicular to the side pad when collision occurs. Because the side-pad is smooth, the collision is perfectly elastic. The impact force given by the side-pad onto the ball just reverses the direction of the perpendicular velocity without cause any change to its magnitude. Thus, the speed of the ball is unchanged after collision, but there is a change of velocity, hence a change of perpendicular momentum. This change of perpendicular momentum resulted in a change of the final momentum of the ball.
有沒有線動量的守恆的例子!?如何判斷!?
When a rigid billard ball collides with another rigid billard ball, momenta of the two balls before and after collision are conserved.

因為桌球反彈後的方向不是原來射入的方向﹐既然速度向量不同﹐故線動量不守恆。
桌球加球檯則符合線動量守恆。