来自《耶鲁大学开放课程：基础物理 第15集 第15集》

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  So this is the initial momentum, that's going to be the final momentum.
  这是初始动量 亦是最终动量
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  That also means initial momentum square is final momentum squared.
  也就是说初动量平方等于末动量平方
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  If two vectors are equal, the squares are equal.
  如果两个矢量相等 其平方值也相等
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  Now, the final momentum squared,
  最终动能平方值
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  I say we can evaluate not in the lab frame,
  我们并不在实验室框架中得出该值
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  but in another frame called the center of mass frame.
  而选择在质心系里得出
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  And here is a center of mass frame.
  我来向大家介绍一下质心系
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  Suppose you start moving to the right at some speed,
  假设你以某速度开始向右移动
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  what do you think will happen?
  你觉得会发生什么
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  Imagine in your head.
  在脑海里想象一下
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  You move to the right, this proton will start moving to the left,
  你向右移动 这个质子会开始向左移动
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  this proton will appear slower.
  而这质子会显得慢一些