来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  from falling through the floor,
  不让你掉下来
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  but also accelerate you counter to what gravity wants to do.
  还要抵消重力的作用让你加速
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  That's why it is g plus a.
  所以加速度是 g 加上 a
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  So, you picked up some speed,
  所以当你加到一定速度
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  then you're coasting along at a steady speed.
  然后你便匀速上升的时候
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  Then, a drops out and you weigh your normal self.
  此时就没有 a 了 你又感觉体重正常
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  As you come to the top of the building,
  当你要到顶时
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  the elevator has to decelerate,
  电梯需要减速
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  so that it loses its positive velocity and comes to rest.
  此时电梯速度减小最后静止
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  So a will be negative
  那么 a 就会变负
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  and w, in fact, will be less than mg.
  w 事实上就会比 mg 小
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  So, you will feel weightless for a short time
  所以你会短时间的感觉到无重力