来自《耶鲁大学开放课程：基础物理 第5集 第5集》

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  because what force are you going to use?
  你会用哪个力呢
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  It was something now, it is something later,
  它现在是这样 晚点又是另一个样
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  it is positive now, it's negative later.
  现在向正方向 晚点向负方向
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  So, we cannot apply this formula because F is not a constant.
  所以我们不能用这条公式因为 F 不恒定
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  So, the usual trick in calculus is find me an interval in x,
  微积分中的惯用方法是取一小段 x
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  which is so narrow, and I'll make it as narrow as you insist,
  很小的一段 你要多小我就取多小
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  so that during that period I'll think that F is a constant
  小到这一段上的 F 可以认为是恒定值
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  and the constant value of F is just the function F at that time x,
  而这个定值 F 就是此时的 x 的函数
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  at that time where the location is x.
  即当位置为 x 的瞬间
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  So, for that tiny interval I can still say that F is constant;
  所以在这个小区间内我还是能说力恒定
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  F over m is some constant over the tiny interval,
  F / m 在这段窄小区间内为恒量
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  then the change in kinetic energy will be then F of x times Δx.
  那么动能变化量就等于 F(x) · Δx