来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  because if you multiply this by m·g,
  因为你可以乘以 m·g
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  well, you just scale the graph by m·g
  这样你就可以按比例 m·g 来改变这个图
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  it looks the same.
  它看起来会是一样的
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  So, take a photograph of a roller coaster,
  对这个过山车照相
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  multiply the height in meters by m·g,
  以米作高度单位并乘以 m·g
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  it's going to still look like the roller coaster.
  它依然看起来像过山车
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  That is my potential energy for this problem
  这就是这个例子中的势能
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  and the claim is that kinetic plus potential will not change,
  我们要求动能加势能的和不变
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  so K + U is a constant;
  也就是 K + U 是一个常量
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  let's call the constant E.
  我们称它为常量 E
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  So, if a trolley begins here at the top,
  那么如果一个过山车从这个最高点出发
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  what is its total energy?
  总能量是多少呢