来自《耶鲁大学开放课程：基础物理 第9集 第9集》

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  A dead snake could be a rigid body.
  死蛇才有可能是刚体
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  A living snake -- definitely not a rigid body.
  一只活着的蛇 绝对不可能是刚体
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  Now, you've got to remember that
  你们要记住
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  I've given you a break by studying rigid bodies,
  虽然我们现在学的是刚体
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  but real bodies
  但生活中的物体
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  can actually not only wobble and twist and turn,
  不但可以摇摆 扭曲 转弯
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  they can also vibrate,
  还可以振动
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  and they can move their arms around.
  可以随便乱动
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  That's much more difficult,
  这样的话问题就复杂了
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  so we won't go there,
  所以我们不考虑
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  and we'll try to take the simplest rigid body.
  只研究最简单的刚体
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  But I said,
  但我也说过了