来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  of a particle moving in the x-y plane.
  质点在 x-y 平面内的运动
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  x and y can be whatever you like,
  你可以任意选取 x 和 y
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  but I picked a very special example
  但我这里讲个特例
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  where x looked like this: i·r·cos ωt + j·r·sin ωt.
  x 为 i·r·cos ωt + j·r·sin ωt
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  You should go back and remember what we did.
  你们应该回忆一下上节课的内容
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  If it took you a while to digest it,
  如果你还要看一会儿才能领会
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  I ask you to think harder.
  你就得好好努力了
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  This describes a particle that's going around in a circle.
  这个式子描述了质点的圆周运动
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  We know it's going around in a circle
  我们之所以知道它做圆周运动
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  because if I find the length of this vector,
  是因为我求出了这个矢量的模长
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  which is the x-square part, plus the y-square part,
  也就是 x 的平方加上 y 的平方
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  I just get r square at all times,
  我就得到了它在任意时刻的模长平方