来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  Anybody have an idea why?
  有人知道原因吗
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  That's correct.
  没错
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  So her answer was, if it was a full parabola,
  她说 如果运动轨迹为一条完整的抛物线
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  then we know it would've been at the ground
  那么它在我们所设定的零时刻之前
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  before I set my clock to 0.
  就已经落地了
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  First of all, negative time should not bother anybody;
  首先 负的时间对我们不存在任何干扰
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  t=0 is when I set the clock,
  t=0使我们设定的初始时间
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  I measured time forward,
  我是用顺序计时
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  but yesterday would be t = -1 day, right?
  所以昨天就可以记为-1天 对吗
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  So we don't have any trouble with negative times.
  所以此处负根对我们不构成干扰
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  So the point is, this equation,
  关键是 这个方程
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  it does not know about the building.
  和楼房并没有关系