A fundamental law of physics holds that momentum is always conserved, meaning that unless some outside force enters a system, its total momentum must stay constant.
物理学一条基本定律认为动量守恒，也就是说，除非外力进入系统，否则总动量一定保持不变。
And if there is no net external force on the system as a whole during the collisions, then momentum is conserved.
若没有外力，作用于系统整体，在碰撞过程中，动量就是守恒的。
And if there is no net external force on the system as a whole during the collisions, then momentum is conserved.
若没有外力，作用于系统整体，在碰撞过程中，动量就是守恒的。
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