来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  you get the second derivative of the center of mass coordinate.
  你就得到了质心坐标的二阶导数
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  So what have I done?
  那么我刚才都做了什么呢
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  I have introduced a fictitious entity,
  我引入了一个虚构的量
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  the center of mass.
  质心
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  The center of mass is a location X,
  质心位于 X 处
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  some kind of a weighted average of x1 and x2.
  就是一种对 x1 和 x2 的加权平均
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  By "weighted", I mean if m1 and m2 are equal,
  所谓"加权" 就是说如果 m1 和 m2 相等
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  then capital M will be two times that mass
  即 M 等于 2·m
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  and you'll just get (x1 + x2) / 2.
  质心就等于 (x1 + x2) / 2
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  The center of mass will sit right in between.
  质心就会正好位于两点的中间
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  But if m1 is heavier,
  但是如果 m1 更重一点
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  it'll be tilted towards m1;
  质心将会靠近 m1