来自《耶鲁大学开放课程：基础物理 第10集 第10集》

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  The cross terms are like the 2A dot B terms.
  交叉项就像 2A 点乘 B
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  So, here is the 2.
  所以这里是系数 2
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  One of the vectors A is the vector d.
  其中一个向量 A 是 向量 d
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  And it's going to be taking the dot product with mi·ri summed over i
  这里是点乘 mi·ri 对 i 的求和
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  because there's going to be a d dot ri twice.
  因为这里有两倍的 d 点乘 ri
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  But d is not dependent on the summation index.
  但是 d 与求和指标无关
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  It's common to all the terms in the sum and you pull it out.
  它对所有项来说都是常量 可以提出来
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  Now, you can think about what this is.
  你现在想想看这是什么
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  This is every mass multiplied by its position.
  这是每一个质点乘以其位置
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  So, take this guy, sum over i, mi·ri,
  取这一项 mi·ri 对 i 求和
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  divide it by the total mass, and multiply it by the total mass.
  再除以总质量 乘以总质量
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  But what is this?
  这一部分是什么