来自《耶鲁大学开放课程：基础物理 第2集 第2集》

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  locally can be approximated as being part of some circle.
  都可以近似地认为是圆周的一部分
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  You can take circles of different size
  你可以做不同大小的圆周运动
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  and place them against your trajectory and see which one fits.
  然后把它们放到你的轨迹上 看哪一个匹配
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  That's the R.
  相匹配的就是要找的 R
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  v^2 / R is the acceleration directed towards the center of that circle.
  v^2 / R 就是指向那个圆心的加速度
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  One miscellaneous result, which we don't use very much now,
  这是一个很复杂的结果 现在并不常用
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  but which I should mention to you is the following.
  但是我要提醒你们注意下面的
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  Suppose this is the ground with some origin here.
  假设这是地面 这里有个原点
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  That's the plane with some origin here.
  这是一个平面 这里是原点
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  And in the plane there is some other object.
  在平面上有一个物体
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  What's the location of the object?
  这些物体的位置怎么表述
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  Let us say P for plane, G for ground, O for object.
  我们定义 P 是平面 G 是地面 O 是物体