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'm going to write it as follows
  我将按下面的形式写出来
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  m1·x1, with two dots on it;
  m1·x1 上面加两个点
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  the two dots telling you it's two derivatives,
  这两个点表示这是二阶导数
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  because one dot is one derivative,
  一个点是一阶导数
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  three dots is three derivatives.
  三个点是三阶导数
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  Of course, at some point this notation becomes unwieldy,
  当然某些情况下这种写法不太方便
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  but you never deal with more than two derivatives,
  但是你不会碰到超过二阶的问题
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  so this works.
  所以这招还是管用的
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  This is m·a, so don't forget the dots, okay?
  这就是 m·a 不要忘记这两个点 好吗
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  This is not some foreign alphabet.
  这不是什么外语字母
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  Every dot is a derivative.
  每个点代表一阶导数
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  You should remember that when I do subsequent manipulation.
  在我稍后推导的时候你们也要牢记这一点