来自《耶鲁大学开放课程：基础物理 第5集 第5集》

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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
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  F = k · x or - k · x is a particular graph,
  F = k · x 或 - k · x 是它的特定图象
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  I'm just taking any function of x that I want.
  这里我只是随手画个我想画的 x 的函数
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  Now, the acceleration is not constant
  现在加速度不再恒定
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  because the force is not a constant.
  因为力不恒定
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  So, if you wait for one hour
  所以如果你等一小时