来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  So, this is called a two-body problem.
  这就是二体问题
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  So, this is one example where you realize, "Hey, center of mass,
  这是一个例子 当你发现 "哇 质心
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  if I follow it, it cannot be moving
  如果我跟踪它 它不会移动
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  and therefore the actual motion of planets
  所以行星的实际运动
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  is more complicated than we thought."
  比我们想象的要复杂得多"
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  Okay, then, there is a whole slew of problems one can do,
  那么 还是有很多问题我们可以解决的
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  where the center of mass is not moving.
  这些问题中的质心都不移动
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  So, I'll just give you a couple of examples; then, I will stop,
  我会给你们一些例子 然后我会结束
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  but I won't do the numbers.
  但我不会做计算
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  Here is one example.
  这里有个例子
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  That is a carriage
  有一个马车
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  that contains a horse,
  里面有一匹马