来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  say the first two -- forget the third one --
  假设是前两个 忘记第三个物体
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  take these two,
  考虑这两个物体
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  find their center of mass,
  找到它们的质心
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  let's call it x12
  我们称其为 x12
• 
  
  and with that total mass M12, which is just m1 + m2,
  总质量为 M12 也就是 m1 + m2
• 
  
  trade these two for a new fictitious object
  把这两个物体看作一个虚构的物体
• 
  
  and put that here, and forget these two.
  把它放在这里 忘记这两个物体
• 
  
  But on that one point it deposits the mass of these two;
  但是在这个点上放置着这两个物体
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  now, you take the center of mass of this object with the third object,
  现在再来考察这个新物体的质心和第三个物体
• 
  
  by the same weighting process,
  使用相同的权重方法
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  m3·x3 + M12·X12 divided by the total mass;
  m3·x3 + M12·X12 除以总质量
• 
  
  you'll get the same answer as here
  你会得到和这里相同的答案