来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  Therefore, x is r, in general.
  通常习惯将 x 写成 r
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  The U that you want for gravitation looks like this.
  万有引力的势能 U 写出来就是这样
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  So, gravitational potential energy
  离质量中心任意远的
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  for problems arbitrarily far from the center of this mass,
  引力势能
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  not near just the surface of the Earth
  不是仅在地表附近
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  but wherever you go, in any gravitational problem,
  而是引力场中任何位置
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  this is U.
  U 都可以这样表示
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  You can check that if you took this U,
  检查一下这个 U 的表达式
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  here's what I'm telling you guys to do.
  也是我要求你们这样做的
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  If you're really care.
  仔细看看
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  Did I miss something?
  我落了什么东西吗
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  No. Let's see.
  不是的 看这里