来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  what should you know from the last lecture?
  你们应该从上节课了解到什么
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  Here's the main point.
  这才是重点
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  The Law of Conservation of Energy
  能量守恒定律
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  is a powerful concept that I told you,
  是我告诉过你们的一个重要的概念
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  survives even the quantum revolution.
  量子革命都没能推翻它
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  So, it looks like we may ascribe to a system
  那么 我们也许应该给一个系统
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  something called the total energy,
  设定一个总能量
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  which is the kinetic energy,
  包括动能
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  which is always (1/2)·m·v^2,
  它恒等于 (1/2)·m·v^2
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  and potential energy which depends on the location of the object,
  还有由物体位置决定的势能
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  and it varies from problem to problem,
  在不同的问题中会有不同的形式
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  depending on whether they're connected to a spring
  物体可能和弹簧相连