来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  I do try to start a few minutes late,
  我已经推迟上课好几分钟了
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  but I also have to finish a few minutes early
  我还得提前下课
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  so you can go to your next class.
  好让你们能赶上下一节课
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  Also, people who come in early maybe should try to
  还有 来得早的人最好尽量
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  sit in the middle part of the classroom,
  坐在教室中间的座位
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  so the latecomers can come in without too much disruption.
  这样就不必站起来给迟到的人让路
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  So, summary of last time.
  总结一下上节课的内容
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  If you live in two dimensions or more,
  如果你要考虑二维或二维以上的问题
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  you've got to use vectors to describe most things.
  大多数情况下需要用矢量来描述
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  The typical vector is called
  表示矢量时一般在
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  V or A or B something with an arrow on it.
  字母 V A 或者 B 上面加个箭头
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  The most important vector is the position vector
  最重要的矢量是位置矢量