来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  will give different numbers.
  积分结果也不相同
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  So, the answer cannot simply be U1 - U2.
  答案不是简单的 U1 - U2
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  First, I'm trying to convince you,
  首先 我在通过随机选取的例子
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  through an example that I selected randomly,
  说服你们
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  that if you took a random force and
  即如果我取任意一个力
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  found the work done along one path, or another path,
  求解它沿着某条或另一条路径所做的功
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  you will in fact get two different answers, okay?
  你实际上会得到两个不同的结果
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  That's the first thing, to appreciate that there's a problem.
  这是第一个方面 注意这里有个问题
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  So generally, if we took a random force,
  一般来说 如果我们取任意一个力
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  not a frictional force,
  摩擦力除外
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  a force that depends only on location and not velocity,
  这个力只取决于位置而非速度
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  it will not be possible to define a potential energy
  这样就既不能定义出势能