来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  that gives you the velocity later.
  那就能得到稍后的速度
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  Take the difference of the two velocities
  求出速度的差值
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  and divide by the difference of the two times,
  除以时间的差值
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  and you've got the acceleration.
  这样就能得到加速度
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  Of course,
  当然
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  you have allowed it to move a finite distance in a finite time.
  你是让它在有限的时间内移动有限的距离
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  What you should imagine doing
  你应该想到的做法
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  is making these three measurements more and more quickly.
  是在尽量短的时间内完成这三次测量
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  You need three positional measurements.
  你需要测量三次位移
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  Now, a little later, and a little later later,
  现在 再晚一点 再再晚一点
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  because between the first and second, you get a velocity,
  因为第一个和第二个位置 你得到一个速度
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  the second and third you get another velocity.
  第二个和第三个位置 你得到另一个速度