来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  that contains a horse,
  里面有一匹马
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  and the horse is on this far end.
  马在这一端
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  And as they tell us to do,
  正如我们之前所知道的
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  we won't draw the horse,
  我们不用把马画出来
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  we'll just say it's a point mass m,
  我们只用一个质点 m 来表示
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  and the railway carriage is a big mass, capital M,
  马车是个大质点 大写 M
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  and let's say the left hand of the railway carriage is here.
  假设马车左边在这里
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  Now, you cannot see the horse, okay? The horse is inside;
  现在你看不到马 它在马车内部
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  the horse decides to -- ; now,
  马现在决定要
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  he said, "I'm tired of sitting on this side of this room,
  它说 "现在我不想呆在马车的这一端了
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  I'm going to the other side." The horse goes to the other side.
  我要到另一端去" 于是马移动到另一端
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  First of all, you will know something's going on
  首先 你们可以不用往里面看