来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  but it's not hard to imagine
  不难想象
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  the left-hand side would be different.
  左边会改变
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  Therefore, in general this will not be true.
  因此 一般来说等式不成立
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  So, even though there's a relation between work done
  所以 虽然对物体所做的功
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  and increase in kinetic energy,
  和其动能增量之间有关联
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  you will not get a Law of Conservation of Energy,
  但不能由此推导出能量守恒定律
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  unless this is true.
  除非这个是正确的
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  Then I said, there is still hope
  但我们还有希望
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  because we can, in fact, manufacture forces for which this is true.
  因为事实上我们能构造一些力使等式成立
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  To manufacture the force,
  为了构造这种力
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  you pick any function U, of x and y that you like,
  任选一个关于 x 和 y 的函数 U
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  then define a new force
  然后定义一个新的力