来自《耶鲁大学开放课程：基础物理 第5集 第5集》

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  It'll still look like m · g but g won't be 9.8,
  它仍会是 m · g 但 g 不再等于 9.8
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  g will become 9.6 and 9.2 and 9.1,
  g 会变成 9.6 9.2 9.1
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  it'll decrease.
  逐渐减弱
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  But the variation of gravity in terrestrial experiments is pretty small
  但在地面附近的实验中
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  so we won't worry about it.
  重力变化小得可以忽略
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  But the force of a spring definitely varies with x in a noticeable way.
  但弹簧弹力明显随 x 变化
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  The force of a spring is one concrete example;
  弹簧弹力是个很具体的例子
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  it's what you all seem to recognize; it's this one .
  看来你们都知道 就是这个
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  So then, we have to ask
  那么 我们就要问
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  what is the Work Energy Theorem when the forces itself vary?
  在力会改变的情况下功能原理会怎么变化
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  So, let's draw ourselves a force which is varying,
  那么 我们来画一个会改变的力
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  here is x, here is my force; it does something.
  这是 x 这是力 它有些变化