来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  the potential energy is then m·g·y and everybody can check.
  势能就是 m·g·y 每个人都可以检验
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  If you take the -y derivative of this, you get the force.
  如果对 -y 求导 你会求得力
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  That's the potential energy.
  这是势能
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  And therefore, when a body is falling near the surface of the Earth,
  因此 一个物体在近地表坠落
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  we may assert that (1/2)·m·v^2 + m·g,
  我们可以说 (1/2)·m·v^2 + m·g
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  let me call it height instead of y,
  我们用高度来代替 y
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  let me call this coordinate h instead of y;
  用坐标 h 代替 y
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  I don't think it matters too much.
  这没什么影响
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  That is the total energy and that is a constant.
  这是总能量 是个恒量
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  We applied it the other day to roller coasters,
  我们曾把它应用于过山车问题
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  going up and down, speeding up and slowing down.
  上下行驶 加速减速
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  It's just that this number will not change.
  但这个数值不会变