来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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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
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  It's got potential energy equal to the height,
  它的势能等于高度
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  it's got no kinetic energy,
  它没有动能
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  so total energy in fact is just its height.
  那么总能量事实上就是它的高度
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  And total energy cannot change as the trolley goes up and down.
  总能量在过山车上下运动的时候不会改变
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  So, you draw a line at that height
  那么你在那个高度的地方画一条线