来自《耶鲁大学开放课程：基础物理 第11集 第11集》

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  in the other case, you add it to L
  另一种情况 初始角动量是 L
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  and conclude that it will rotate.
  最后它就会旋转
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  So, the gyro is constantly trying to fall down but it's chasing itself
  所以陀螺试着在下落同时 追逐着自己
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  Just like this is constantly trying to fall down and chasing itself
  和这个一样 它要下落 又围追自己
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  So, the last thing I wanted to calculate
  那么我要计算的最后一个物理量
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  is the rate at which it goes round and round.
  就是它转圈圈时的速率
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  That's a pretty easy calculation.
  这个非常容易计算
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  I will do that and I will let you go.
  我讲完这个 就放你们走
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  So, let's ask the following question
  那么 提出下面一个问题
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  Here is the gyro whose angular momentum was L.
  假设一个陀螺 其角动量为 L.
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  A little later L+ΔL looks like that.
  之后某一时间变为 L+ΔL 像这样
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  This is ΔL.
  这是 ΔL