来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  in the end, you can measure what you can say is the acceleration now.
  最终 你会测量出你所说的现在的加速度
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  That's the meaning of the limit in calculus.
  那是微积分学里的极限思想
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  You take Δx and you take Δt.
  你得到 Δx 和 Δt
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  What's the meaning of Δx and Δt going to 0?
  Δx 和 Δt 都趋近于零意味着什么呢
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  It means, measure them as quickly as you can.
  意味着 在尽量短的时间内测出这些值
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  In the real world, no one's going to measure it instantaneously,
  在现实生活中 没人能一瞬间测出来
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  but we can make the difference as small as we like.
  但我们可以使差值越来越小
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  Mathematically, we can make it 0.
  在数学上讲 我们把它变成零
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  In that limit, we can measure velocity right now.
  取极限的话 我们可以马上测出速度
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  That means we can also measure velocity slightly later
  那我们也可以测出稍微迟一点的速度
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  and make the slightly later come as close to right now as we want.
  让那个速度接近我们前面测得的速度
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  Then, that ratio will give you acceleration.
  那么 比值就等于加速度