来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  If I change the force law to 1/r to the 2.1,
  但如果定律中 r 的指数变成了 2.1
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  then it's a different story.
  那就完全不一样了
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  Anyway, so we want to solve it but we cannot do the ellipse;
  现在我们来解这个方程 但不能用椭圆
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  it's too hard.
  这太难了
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  So, we are going to do the circle.
  我们用圆来解
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  We're going to take the orbit to be circular,
  假设轨道是圆的
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  and we're going to apply this force law to that
  把定律应用在这种情况下
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  and see what we can get.
  看看我们会得到什么结果
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  So, let's do that.
  我们就这么做
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  We can ask ourselves,
  可以问问自己
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  "Can a planet have a circular orbit of radius r?"
  "行星可能会有半径为 r 的圆形轨道吗"
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  We go back to F = m·a.
  我们回到 F = m·a