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 if you have a body like this,
  但如果你拿一个像这样的物体
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  it's not enough to tell me where it is,
  光告诉我它的位置是不够的
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  because if I throw this up in the air,
  因为当我把它抛向空中的时候
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  you can imagine that
  你可以想象
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  as it travels along some parabolic path,
  当它沿着某条抛物线运动时
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  one instant the eraser may look like this,
  在某一时刻 黑板擦可能是这样的
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  the next instant and the eraser may look like this.
  下一时刻它可能又是这样的了
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  So, we say that it is translating and rotating.
  所以 我们说它既有平动 又有转动
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  It's a non-obvious theorem that
  很多人都无法理解
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  anything the body does from one instant to the next instant
  物体在某一时间段内的任何运动
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  can be achieved by first translating it,
  都可以通过先平动
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  then doing a slight rotation
  再转动