来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  and the y will be the y derivative.
  y 分量就是函数对 y 的偏导数
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  That's not obvious,
  这不是显而易见的
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  but in other words it's clear it's sufficient
  但它既是充分的
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  but it's also necessary.
  也是必要的
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  So, every conservative force
  所以 所有的保守力
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  is the derivative of some potential, in this fashion.
  都是某个势函数的导数 像这样
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  Finally, I said if someone comes and gives you a force
  最后 我说如果有人过来给出一个力
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  and says, "Is it conservative or not?"
  然后问 "这是保守力吗"
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  Either you can think in your head
  你们可以动脑筋想想
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  and try to guess that function U,
  试图找到函数 U
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  of which this F is the derivative,
  F 是它的导数
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  in this fashion.
  形成这样的形式