来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  track, mg and they're all acting down.
  轨道 重力 m·g 作用的方向都向下
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  So you see,
  所以你们看
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  that's when you get a little worried.
  这就是你们所担心的问题
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  Okay, this guy's up there,
  它在上面
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  all the forces are down.
  但所有的力都朝下
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  So, we got to ask, so what does that mean?
  我们要问 这意味着什么呢
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  Well, I have to agree they're all down.
  我很赞同他们都朝下
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  I say T + m · g is the downward force,
  T + m·g 是向下的力
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  that's got to be m · v^2 / r.
  等于 m·v^2 / r
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  Therefore, T = m · ( g -
  因此 T = m·( g -
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  Oh, I'm sorry, ( v^2 / r - g )
  抱歉 ( v^2 / r - g )
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  So, what does that mean?
  这是什么意思呢