方向为x正轴方向，在碰撞前，它有一定动量。
Let this be the direction of increasing x, and so before the collision I have a certain amount of momentum.
当处理碰撞问题，物体的速度，在碰撞前无撇号。
When we deal with collisions, the velocities of the objects before the collision are unprimed.
碰撞前没有动能。
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