来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  the balloon goes one way, the air goes the other way,
  气球与喷出的气体向相反方向运动
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  action and reaction are equal, even lay people know that.
  作用力等于反作用力 地球人都知道
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  Or, if you stand on a frozen lake
  或者 假如你站在一个冰冻的湖上
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  and you take a gun and you fire something,
  你用枪射击某物体
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  but then the bullet goes
  当子弹飞出时
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  one way and you go the opposite way, again,
  你会朝相反方向运动
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  because of Conservation of Momentum.
  由动量守恒可以得知
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  The rocket is a little more subtle
  火箭是十分精细复杂的
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  and I just want to mention a few aspects of it.
  我只想介绍它的几个方面
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  I don't want to go into the rocket problem in any detail.
  我不想详细地讨论火箭的问题
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  It's good for you to know how these things are done.
  你们知道它的原理是有好处的
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  Here's a rocket whose mass at this instant is M,
  这是一个火箭 它在这一瞬间的质量为 M