来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  If you don't have the units,
  如果没有的话
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  then if you say the answer is 19,
  比如你的答案是十九
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  then we don't know what it means.
  那我们就不知道这是什么意思
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  At a given instant, it's got a location.
  在一个给定的时刻 它在某一位置
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  So what we would like to do is to describe what the object does
  然后我们要做的就是描述这个物体的运动
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  by drawing a graph of time versus space
  通过画一个"x-t"图
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  and the graph would be something like this.
  画出来的图就像这样
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  You've got to learn how to read this graph.
  你需要学习如何看图
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  I'm assuming everyone knows how to read it.
  我要求你们每个人都要会看这种图
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  This doesn't mean the object is bobbing up and down.
  它不表示物体在上下摆动
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  I hope you realize that.
  希望你们能注意这一点
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  Even though the graph is going up and down,
  虽然图上曲线是上下波动