来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  A rocket is something everybody understands
  大家都知道火箭
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  but it's a little more complicated than you think.
  但它比你想象的要复杂一些
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  Everyone knows you blow up a balloon,
  每个人都知道你吹起来一个气球
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  you let it go,
  然后放掉气
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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