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, let me say in words what I'm saying.
  让我总结一下我讲过的东西
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  Take a collection of bodies.
  取一组物体
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  At a given instant everything is moving;
  在一给定的时刻 它们都在运动
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  it's got its own velocity and its momentum,
  它们具有各自的速度和动量
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  add up all the momenta.
  对所有的动量求和
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  If you had one dimension, just add the numbers.
  如果是一维运动 只需要对数字求和
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  If in two dimensions, add the vectors;
  如果运动是二维的 将速度相加
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  you get a total momentum.
  就得到总动量
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  That total momentum does not change
  如果没有外力作用的话
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  if there are no outside forces acting on it.
  总动量不会改变
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  So, a classic example is two people are standing on ice.
  一个经典的例子是 两个站在冰上的人
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  Their total momentum is 0 to begin with,
  他们的初始动量为零