来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  So, what you really should do is
  所以你应该做的是
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  divide the Earth into tiny pieces,
  把地球分为小微元
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  each one of which is point-like,
  每一个都视作点
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  find the force on each-- from each chunk of the Earth,
  算出地球上每一块的力
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  using this law, and add it up.
  使用这个公式 加起来
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  And if you're lucky, it will look as if
  幸运的话 结果会看起来好像
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  all the pull is coming from one point at the center,
  所有拉力来自一个中心
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  carrying the entire mass of the Earth.
  承载着整个地球的质量
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  So, what branch of mathematics
  你必须使用数学中哪一个分支
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  do you have to use to get that result? Yes?
  来计算这个问题 请讲
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  What branch of mathematics? What kind of calculus?
  数学的哪个分支 哪种积分
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  Had it been invented?
  当时有了吗