来自《耶鲁大学开放课程：基础物理 第2集 第2集》

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  You say this is a commutative law.
  这就是交换律
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  It doesn't matter the sequence in which you add the two vectors.
  在做两个矢量的加法时无所谓相加的顺序
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  You know that's true for ordinary numbers, right?
  这对于普通数字的加法成立 不是吗
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  3 + 4 and 4 + 3 are the same.
  3 + 4 和 4 + 3 结果一样
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  It's also true for vectors.
  它对矢量同样也成立
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  But the law of composition is not always commutative.
  但是两两结合的运算并不总是可交换的
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  There are certain occasions
  在有些特定的情况下
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  in which you first do A and then do B;
  你先做 A 再做 B
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  that's not the same as first doing B and then doing A.
  和先做 B 再做 A 是不一样的
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  We'll come to that later.
  我们以后会讨论那个问题
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  Right now, this is the simple law.
  目前只讨论简单情形
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  Alright, next thing I want to do is to define the vector
  接下来我想做的是定义一个矢量