来自《耶鲁大学开放课程：基础物理 第1集 第1集》

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  is not enough to tell you where the particle will be.
  是不足以告诉你它接下来的运动的
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  For example,
  比如说
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  let's take the case where the particle is falling under gravity.
  举个例子 一个质点正在重力的作用下下落
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  Then you guys know,
  然后你们都知道
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  you just told me, acceleration is -9.8,
  你告诉我 加速度是-9.8
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  We call it "minus" because it's accelerating down
  我们取负号是因为加速度方向朝下
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  and up was taken to be the positive direction.
  而正方向为竖直向上
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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