来自《耶鲁大学开放课程：基础物理 第13集 第13集》

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  So, the velocity of light will always come out to be the same.
  所以光速永远都是不变的
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  That is built into the formula but it is a good thing to test.
  它包含在公式里 但检验一下也不错
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  So, what you find is, velocities don't add in the simple way.
  可以看出 速度不是那么简单叠加的
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  If they did, you are in trouble.
  如果是那样 麻烦就来了
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  You cannot get an upper limit if they added in the simple way
  如果速度叠加这样容易 就没有上限了
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  because you can put a rocket inside another rocket
  因为可以把一个火箭装在另一个火箭了
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  inside another rocket and add up all the speeds
  再装进其他火箭里 把所有速度加起来
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  and even though each one is less than 0,
  即使每一个速度都小于零
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  the total could be whatever you like.
  它们的和也可以是任何数字
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  But they don't add that way.
  还好速度不是那样叠加
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  They add this way so that the answer is either bigger
  而是这样的 所以如果 v 和 u 方向相反
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  than what you think if v and u are opposite,
  那么答案就会大一点