来自《耶鲁大学开放课程：基础物理 第15集 第15集》

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  and you've got to think about why that is not the answer.
  你一定要好好想想为什么这不是答案
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  Why is that wrong?
  为什么它是错的
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  Why doesn't the photon energy turn into my extra mass?
  为什么光子能量没有转变成我的额外质量
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  Yes?
  请讲
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  In other words, when I absorb the photon,
  换句话说 当我吸收了光子
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  I also have to recoil to conserve the momentum of the photon.
  我的反冲保存了光子的动量
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  Consequently, my final energy
  因此 我最终的能量
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  will equal old energy plus photon energy,
  等于原来的能量加光子的能量
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  but that does not mean all of it is rest energy,
  但它并不是全都成了静止能量
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  because there's some energy of motion.
  因为有动能
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  So, you've got to make sure that in the end you calculate it
  你必须确定最终你把它计算在内
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  so that the total energy is balanced,
  保证总能量守恒