来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  I take the total mass and divide it by L;
  我取总质量然后除以 L
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  that's the mass per unit length.
  这就是单位长度的质量
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  And this fellow has a width dx,
  这一小段的长度是 dx
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  so the mass of this little sliver is M / L · dx.
  那么它的质量就是 M / L · dx
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  Therefore, the center of mass that I want
  因此 待求的质心位置
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  is found by taking this sliver of that mass,
  就是通过这些小片段质量
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  multiplying by its coordinate and summing over all the slivers,
  乘以它的坐标然后对所有的片段求和得到的
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  which is what we do by the integral from 0 to L.
  也就是从 0 到 L 的积分
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  Then, I should divide by the total mass, which is just M.
  然后我应该除以总质量 M
• 
  
  You can see now if you do this calculation.
  你们现在可以看到如果你做了这样一个运算
• 
  
  I get 1 / L;
  提出 1 / L
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  then, I get integral x·dx from 0 to L,
  然后我对 x·dx 积分 从0积到L