来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  In that case, y will be -gt^(2)/2+c+bt.
  在这个情况下y=-gt^(2)/2+c+bt
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  So，the point is,every object falling under gravity
  关键是 每一个受重力作用下落的物体
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  is given by the same formula,
  都有同样的表达式
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  but there are many, many objects that can have many histories,
  但是各个受重力作用下落的物体
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  all falling under gravity,
  都有不同的初始条件
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  and what's different from one object and the other object is,
  所以它们的区别在于
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  when was it dropped,
  下落时刻
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  from what height, and with what initial speed.
  下落高度和初始速度
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  That's what these numbers are going to tell us
  这些系数能告诉我们这些信息
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  and we can verify that as follows.
  下面我们开始验证这一点
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  If you want to know what the number c is,
  比如 如果你想知道系数c代表的是什么
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  you say, let's put time t=0.
  我们令时间t=0