来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  So, the momentum of the center of mass
  那么 这个整体的质心的动量
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  is the sum of the momentum of the two parts,
  就等于两部分的动量之和
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  but what you're learning is --
  你们要会的东西是
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  so let me write it one more time -- if F external is equal to 0,
  让我再写一次 如果外力 F 等于零
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  then p1 + p2 does not change with time.
  那么 p1 + p2 就不随时间而变化
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  This is a very, very basic and fundamental property,
  这是一条非常非常基本的性质
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  and it's in fact another result that survives all the revolutions
  实际上 这是另一条能经得住
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  of relativity and quantum mechanics,
  相对论和量子力学考验的结论
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  where what I've said for two bodies
  我讲的这些有关两个物体的内容
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  is true for ten bodies;
  同样适用于十个物体
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  you just do the summation over more terms.
  你们只需将更多项相加
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  So, let me say in words what I'm saying.
  让我总结一下我讲过的东西