来自《耶鲁大学开放课程：基础物理 第10集 第10集》

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  it has also finished one full revolution.
  它也正好转了一圈
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  For example, if every part of that circle was colored with some chalk,
  举个例子 如果用粉笔把这个环涂上颜色的话
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  then when it finished the full revolution,
  那么当它完成转动一圈
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  every part of the circle would have touched the road
  环的每一部分就会与路接触
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  and made an impression.
  留下一个痕迹
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  And the length of that thing should be the circumference of the circle.
  这个痕迹的长度就是这个环的周长
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  That's when it rolls without slipping.
  这个例子就是无滑动地滚动的情况
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  So, what does it mean to roll without slipping?
  那么 无滑动的滚动意味着什么呢
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  What's the velocity?
  滚动的速度又是多少
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  But let's calculate the velocity when you roll without slipping.
  让我们来计算无滑动地滚动时的速度吧
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  When you roll without slipping,
  当它无滑动地滚动时
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  the velocity of the center of mass,
  它质心的速度