来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  but we have assumed the mass per unit length,
  但我们已经假定单位长度的质量
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  namely M or this is a fixed number,
  也就是 M 或者这一项是个确定的数值
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  M / L, and then this is the answer.
  M / L 那这就是答案
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  But there are other ways to get this result without doing all the work.
  但还有其它方法来计算 不用这么复杂
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  So, we would like to learn that other method
  那么我们将学习另一种方法
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  because it'll save you a lot of time.
  因为它会节约你们很多时间
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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