来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  On the right-hand side,
  在等式右边
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  you are told to add F·dr for every tiny segment.
  需要把每一小段的 F·dr 相加
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  Well, that is written symbolically as this.
  用这样的符号表示
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  This is the notation we use in calculus.
  这是我们在微积分中用的符号
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  That just means, if you want to go from A to B along some path,
  它代表 如果你沿某路径从 A 到 B
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  you chop up the path into tiny pieces.
  将路径分割为小片段
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  Each tiny segment,
  每个小片段
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  if it's small enough to be approximated by a tiny vector dr,
  只要足够小到可以近似为微小向量 dr
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  then take the dot product of that little dr,
  然后用向量 dr
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  with the force at that point,
  和在这一点的力做点乘
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  which means length of F, times the length of the segment,
  等于 F 的模长乘以片段的长度
• 
  
  times cosine of the angle;
  乘以它们夹角的余弦