来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  So v of t, v is the limit,
  t点的速度等于一个极限
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  Δt goes to 0 of Δx over Δt
  令Δx/Δt中的Δt趋近于0
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  and we use the symbol dx/dt for velocity.
  我们用符号dx/dt表示速度
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  So technically, if you ask what does the velocity stand for--
  从学术角度来说 如果你问速度表示什么
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  Let me draw a general situation.
  那让我来给你画一个常见的场景
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  If a particle goes from here to here, Δx/Δt,
  如果质子从这里运动到这里 Δx/Δt
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  I don't know how well you can see it in this figure here,
  我不清楚你们对于这个图像的理解
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  is the slope of a straight line connecting these two points,
  这条连接两点的斜线
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  and as the points come closer and closer,
  随着两点距离逐渐缩小
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  the straight line would become tangent to the curve.
  两点连线会变成曲线的切线
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  So the velocity at any part of the curve
  曲线上任意一点的速度
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  is tangent to the curve at that point.
  都在该点和曲线相切