来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  That's wrong because you cannot use
  因为你不能应用
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  the Law of Conservation of Energy in this process
  能量守恒定律 在这个过程中
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  when I tell you that it's
  因为我告诉你 这个过程中
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  a totally inelastic collision in the middle.
  发生了完全非弹性碰撞
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  Because, what'll happen is,
  所以 将要发生的是
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  some energy will go into heating up the block;
  有一些能量将加热木块
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  it might even catch fire if the bullet's going too fast.
  假如子弹速度太大 木块甚至还会着火
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  But you can use the Law of Conservation of Momentum all the time
  但是任何时候你都可以使用动量守恒定律
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  in the first collision to deduce that M + m
  在第一个碰撞过程中来算出 M + m
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  times some intermediate velocity is the incoming momentum.
  乘以某个中间时刻的速度得到后面时刻的动量
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  You understand that?
  你们明白了吗
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  From that, you can find the
  从此 你可以求出