来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  What a particle tries to do generally is some crazy thing
  一个质点的运动一般非常混乱
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  which doesn't have a name,
  因此对应的函数没有名字
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  but it's a function x of t.
  但它还是函数x(t)
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  So you should know when you look at a graph like this
  因此当你看到这样一个图表时
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  what it's doing.
  要知道它是怎样运动的
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  So, the two most elementary ideas you learn
  你们学过的两个最基本的概念是
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  are what is the average velocity of an object,
  物体的平均速度
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  as then ordered by the symbol v-bar.
  用"v拔"表示
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  So, the average is found by taking two instants in time,
  平均速度就是取两个时刻
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  say t1 and later t2,
  比如t1和之后的t2
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  and you find out where it was at t2
  用t2时刻的位移
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  minus where it was at t1
  减去t1时刻的位移