来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  Then I'm going straight up, okay.
  然后直着上来
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  So, let's find the work done when I go this way.
  让我们计算一下力在该路径做的功
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  So again, you should be thinking all the time.
  再一次强调 你们应该时刻保持思考
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  You should say if this guy got struck by lightning can I do anything,
  如果讲台上这个人被雷劈了 该怎么办
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  or am I just going to say
  还是你要说
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  "Well, I don't know what he was planning to do."
  "我不知道老师接下来打算干什么"
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  You've got to have some idea what I'm going to do.
  你们最好能猜到我下来要讲什么
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  I'm going to integrate F·dr,
  我现在要计算F·dr的积分
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  first on the horizontal segment, then on the vertical segment.
  先是水平的一段 然后是竖直的一段
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  F is some vector you give me at the point (x, y);
  F 是一个给定的在 (x, y) 点处的矢量
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  I'll plug in some numbers,
  然后代入数值
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  I'll get something times i and something times j.
  得出什么乘以 i 加上什么乘以 j