来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  In 3D, every vector
  在三维里 每个矢量
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  has some amount of i and some amount of j,
  都是有某个 i 的值和某个 j 的值
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  that's, what's true for f, that's true for a.
  F 和 a 同样也是
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  You write it out.
  都要写出来
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  And I told you, if two vectors are equal,
  我也说过 如果两个向量相等
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  then the x components have to match and the y parts also have to match.
  那么它们的 x 分量以及 y 分量都得相等
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  You can see that.
  大家能理解
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  If I draw two arrows on the plane
  假如我在平面上画两个矢量
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  and I tell you this arrow is the same as that arrow,
  我告诉这个两个矢量是相同的
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  it's the same.
  道理是一样的
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  It's got the same horizontal part and the same vertical part.
  它们有相同的水平和垂直分量
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  So, this is really two equations,
  所以这实际上是两个方程