来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  That also cannot happen,
  这也不可能发生
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  so they're moving with the same acceleration.
  所以它们以相同的加速度运动
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  There's only one unknown a.
  所以只有一个未知量 a
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  Once you got this,
  一旦写出了这两个式子
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  you realize what you got to do.
  你就知道该怎么计算了
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  This equation is begging you to be added to this equation.
  把这个方程加到这个方程上去
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  You got a plus f and a minus f,
  这里有一个正 f 和一个负 f
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  so you got to do what you got to do.
  所以你知道该怎么办
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  You add the two,
  把两个方程相加
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  you get 10 = 5a and you get a = 2.
  你就会得到 10 = 5a 算出 a = 2
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  Once you got a = 2,
  一旦你算出 a = 2
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  you can go back and realize here that f = 4,
  你可以回到上面来 求出 f = 4