来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  then you don't have to go through the whole process
  那么你不用从头到尾再来一遍
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  of finding the time at which it turned around.
  来弄清质点何时转向
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  I don't know where that is,
  我不知道我写在哪儿了
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  that disappeared on the blackboard,
  已经不在黑板上了
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  then putting the time equal to 1 second into this formula.
  然后把时间等于1秒代入方程
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  If the question of time is not explicitly brought up,
  如果时间的问题并没有明确提出
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  then you should know that you have to use this formula.
  那你应该知道 必须要用这个式子
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  So how do we get it here?
  那么我们这么到这一步的
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  Well, we say at the top of the loop,
  在曲线顶端
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  when it goes up and comes down the velocity is 0.
  当物体上升 然后速度为0时候
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  Therefore, you say 0^(2)=v0^(2)-2g,
  因此 你得到0^(2)=v0^(2)-2g
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  that's my acceleration,
  这里是加速度