来自《耶鲁大学开放课程：基础物理 第16集 第16集》

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  Suppose you suspend a rod from the ceiling like this.
  假设从天花板上吊一跟小棍
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  It's hanging there. If you give it a twist,
  它吊在那 你扭它一下
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  then it'll go twisting back and forth, like that.
  它就会来回扭动
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  That's also a simple harmonic motion,
  这也是一种简单的谐振子运动
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  except what's varying with time is not the linear coordinate,
  只是变化的不是线性坐标
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  but the angle θ by which the rod has been twisted.
  而是小棍扭动的角度 θ
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  What's the equation in this case?
  此时的方程是什么呢
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  The equation here is I·d^2 θ/dt^2.
  就是 I·d^2 θ/dt^2
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  You guys remember now, that's the analog of m·a?
  这部分就类似于 m·a
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  That's going to be equal to the torque.
  应当等于扭矩
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  Now, what'll happen in the problems we consider
  在我们考虑的问题中
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  is that θ is a small number, and the torque will be
  θ 是个小量 扭矩的作用