来自《耶鲁大学开放课程：基础物理 第11集 第11集》

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  But I want you to think about the following fact.
  但我想让你考虑下面的情况
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  Whenever you take a rigid body that's made up of many bodies,
  任取一个由许多物体组成的刚体
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  each one of them has angular momentum Iω.
  每个物体都有角动量 Iω
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  You added them all up.
  把它们都加起来
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  But as a vector, each one has an angular momentum
  作为一个矢量 每一个都有角动量
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  coming out of the blackboard or going into the blackboard.
  方向是从黑板向外或向内的
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  Because if r and p lie in the xy plane,
  因为如果 r 和 p 位于 xy 平面上
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  then r cross p lies along the z axis, either out or in.
  那么 r 叉乘 p 沿着 z 轴 方向为向外或向内
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  That is why both torque and angular momentum
  这就是为什么转矩和角动量
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  were treated as scalars and not as vectors.
  都被视为标量而不是矢量
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  Because if a vector is constrained to lie up or down the z axis,
  因为如果矢量被约束在 z 轴向上或向下的方向
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  we can forget about the fact it's a vector.
  我们就能忘记它是一个矢量