来自《耶鲁大学开放课程：基础物理 第9集 第9集》

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  So, I'm going to imagine rigid bodies,
  我们现在来设想一些刚体
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  which are living in the plane of the blackboard.
  它们都位于黑板平面内
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  Now, one way to do that is to take a piece of metal,
  你可以拿一片金属
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  just cut out a shape,
  随便切下什么形状
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  and that's a rigid body.
  就有了一个刚体
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  And of course this has a slight thickness,
  当然现实中它肯定会有一点厚度
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  because anything you make of matter has some thickness,
  因为无论什么物质做的东西都会有厚度
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  at least one atom thick,
  至少一个原子的厚度
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  but imagine that thickness is negligible,
  假设这个厚度是可忽略的
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  so it's like a piece of metal,
  所以它就像一块金属
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  a piece of foil or something, but it's rigid,
  一块金属箔或者别的什么 而且还是刚体
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  and this is the rigid body we're going to study.
  这就是我们要研究的刚体