来自《耶鲁大学开放课程：基础物理 第10集 第10集》

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  Angular momentum is conserved if
  满足以下条件时角动量守恒
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  That's like saying linear momentum is conserved
  类似于线动量守恒的条件
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  if the external force is zero.
  外力为零
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  So, let's take some examples of this.
  看看下面几个例子
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  Here's one example.
  这是一个例子
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  There is a disk on a spindle that's spinning around.
  一个圆盘在轴上旋转
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  The disk has some moment of inertia I1,
  这个圆盘具有转动惯量 I1
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  spinning at some angular velocity ω1.
  以角速度 ω1 自转
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  On top of this is a second disk,
  在上方有第二个圆盘
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  just one millimeter away, and it's at rest.
  相距一毫米 处于静止
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  And it's got some moment of inertia I2,
  转动惯量为 I2
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  but it's got the same center as this guy.
  中轴相同