来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  don't use energy conservation from start to finish.
  不要从头到尾都用能量守恒
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  Use momentum conservation,
  用动量守恒
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  find the speed of the composite object.
  找出整体的速度
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  This is what you've got to understand in your head.
  这是你必须要理解的
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  It's not this equation.
  而不是这个方程
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  When can I use Conservation of Mechanical Energy?
  什么时候我可以用机械能守恒定律
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  When can I not?
  什么时候不能
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  A bullet driving into a chunk of wood,
  一颗子弹射入一块木头
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  you better know you cannot use Conservation of Kinetic Energy.
  你最好知道你不能应用动能守恒
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  But once the combination is going up,
  但是一旦整体开始向上摆动
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  trading kinetic for potential, you can.
  仅有动能与势能相互转化 你就可以用它了