来自《耶鲁大学开放课程：基础物理 第9集 第9集》

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  but now we're going to enlarge our study
  但今天我们要把研究范围扩大到
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  to objects which have a size and which have a shape,
  有大小和形状的物体
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  therefore, which have an orientation.
  也就是有指向的物体
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  It's not enough to say the football is here.
  光说明一个橄榄球的位置是不够的
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  The football could be here,
  它可以在这里
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  but its long pointy end could be up to down or side to side.
  但你不知道它的两个尖头是冲着上下还是左右
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  So, what we want to do
  我们想做的是
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  is to break the problem into two parts.
  把这个问题分解成两部分
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  We want to focus on a body which is only rotating but not translating.
  我们先研究只有转动没有平动的物体
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  Once we've sharpened our skills,
  等我们弄清楚了
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  we will then put back the ability of the body to translate.
  我们就可以把平动加进来
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  So, we're going to focus initially on a body that cannot translate.
  所以我们先来研究没有平动的物体