来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  it doesn't add up to the same number.
  它们加起来不是相等的
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  So, we sort of know what the answer should be.
  我们大概知道答案是什么了
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  We know that the thing that cannot move
  我们知道那个不能运行的东西
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  is not the Sun and it's not the Earth.
  不是太阳 它也不是地球
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  It's the center of mass; that's what cannot move.
  是质心 这个不能移动
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  If originally it was at rest, it'll remain at rest.
  如果开始时它是静止的 它会一直静止
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  So, the center of mass, if it cannot move,
  所以 质心如果不能移动
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  so let's start off the Sun here, start off the Earth here,
  我们让太阳在这里 地球在这里
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  join them, the center of mass is somewhere here.
  连起来 质心在这里
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  Actually, the Sun is so much more massive than the Earth,
  实际上 太阳比地球质量大得多
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  the center of mass lies inside the Sun.
  质心在太阳内部
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  But I'm taking another Solar System where
  我们考虑另一个太阳系