来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  or maybe you are not clever enough in finding that function.
  也可能是你们在求函数方面脑子不够灵
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  So, here is a recipe that works for all of this.
  对于这个问题我们有一个万能解法
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  The recipe is: take the force,
  这个方法就是 对这个力
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  take the d by dy of Fx,
  取 Fx 对 y 的偏导数
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  and compare it to d by dx of Fy,
  和 Fy 对 x 的偏导数作比较
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  and if they're equal, it's a conservative force.
  如果它们相等 力就是保守力
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  That's the summary of last lecture.
  这是上节课的总结
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  How much of a detail you want to know
  你们想了解多少细节
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  is entirely up to you.
  就完全随你们自己愿意了
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  Okay, so now--so I'm going to--
  现在 我要开始
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  This is residual from last time.
  这是上次剩下的内容
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  So, I'm going to basically hide that.
  所以 我基本上要忽略掉