来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  It's an average, it doesn't give you enough detail.
  它只是平均值 不能提供足够的细节
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  So it's useful to have the average velocity.
  平均速度还是很有用的
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  It's useful to have the average acceleration,
  平均加速度也是很有用的
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  which you can find by taking similar differences of velocities.
  你可以用类似方法 取速度的变化量求出来
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  But before you even do that,
  但在这之前
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  I want to define for you an important concept,
  我要给你们定义一个重要的概念
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  which is the velocity at a given time, v .
  也就是给定时刻的瞬时速度v
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  So this is the central idea of calculus, right?
  这是微积分的核心思想 对吗
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  I am hoping that if you learned your calculus,
  我希望你们学习微积分时
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  you learned about derivatives
  通过观察x-t图来
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  and so on by looking at x versus t.
  学习导数等内容
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  So, I will remind you, again, this is not a course in calculus.
  我要再次提醒你们 这不是微积分课