来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  It must be clear to most people
  显而易见 我们都知道
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  that the center of mass of this rod is at the center.
  这根棍子的质心就在中点上
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  But how do we argue that?
  但我们如何证明呢
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  How do you make it official?
  你们如何让这成为正式的结论
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  If you do the integral you will get the answer,
  如果积分的话马上就可以得到结果
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  but I want to short circuit the integral.
  但是我想跳过积分这一步
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  And here is a trick.
  窍门在这里
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  It's not going to work for arbitrary bodies.
  当然 并不是对任何物体都能用的
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  If I give you some crazy object like this,
  如果我给你一个这么诡异的物体
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  you cannot do anything.
  你就没辙了
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  But this is a very symmetric object;
  但这个却是一个非常对称的物体
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  you can sort of tell if I take the midpoint,
  你们大概可以看出来如果我取了中点