来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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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
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  or you'll feel your weight is little.
  或者感到自己变轻了
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  Then, you come to rest and the opposite happens on the way down.
  随后静止 下降过程就是与上升过程相反的情况
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  Let me just briefly look at the ride on the way down.
  让我简单的来讲解一下下降过程
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  As you start on the top and go down,
  当你在顶楼准备下降的时候
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  your acceleration is negative.
  你的加速度是负的
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  Remember, you've got to keep track of the sign of acceleration,
  记住 一定要注意加速度的符号