来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  the part, say, due to a spring and the part due to friction.
  一部分关于弹簧 另一部分关于摩擦力
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  The part due to a spring will integrate to give you this.
  弹簧部分的积分得到这个结果
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  Then, you'll still have the part due to friction left over.
  然后 还要加上摩擦部分
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  In that case the result will look like
  这种情况下
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  Instead of being zero,
  不再等于零
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  it will be the work done by friction.
  而等于摩擦力所做的功
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  We write it symbolically this way
  我们象征性地将它写成这样
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  but the frictional force is not a function of x alone,
  但摩擦力不只是 x 的函数
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  but if you knew which path you're taking and which way you're moving,
  但如果知道所选择的运动路线
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  you can do that integral.
  就可以求出积分
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  You want to do this in higher dimensions.
  我们再来考虑更高维度的情形
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  In higher dimensions,
  在更高维度下