来自《耶鲁大学开放课程：基础物理 第2集 第2集》

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  What do I expect of -A?
  我希望 -A 是什么样的呢
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  I expect that if I add -A to A,
  我希望如果用 -A 加上 A
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  I should get the guy who plays the role of 0 in this world,
  应该得到一个零矢量
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  which is the vector of no length.
  也就是一个没有模长的矢量
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  It's very clear that if you want to take A and ask,
  显然 你们会拿 A 来问
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  "What should I add to A so I get the null vector?"
  "拿什么来加矢量 A 可以得到零矢量呢"
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  it's clear that you want to add a vector that looks like that,
  显然应该给它加上这样一个矢量
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  because then you go from the start of this to the finish of that,
  因为从这个矢量的起点指向那个矢量的终点
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  you end up at the same point
  最终指向的是同一个点
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  and you get this invisible 0 vector.
  你就得到这个不可见的零矢量
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  So the minus vector is the same vector flipped over,
  因此负矢量即为同一矢量的翻转
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  pointing the opposite way.
  指向相反的方向