来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  because it's going at a constant speed.
  因为其速度恒定
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  If it goes that far in one week,
  如果它一个星期运动这么远
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  it'll go that far in one week
  下一星期还运动这么远
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  and everyone can tell the area swept out is constant.
  很明显 扫过的面积恒定
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  So, the only thing left to knock off is a third assumption,
  最后一个需要解决的问题是第三个假设
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  the third observation of Kepler involving the time period.
  开普勒观察到的关于周期的第三个规律
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  So, I do the simplest thing.
  我的做法很简单
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  I say, the velocity of this planet is
  行星运动的速度是
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  2·π·r divided by the time period.
  2·π·r 除以周期
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  I put that in here and I get
  代入这里得到
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  or I find r^3 / T^2 is equal to--
  我发现 r^3 / T^2 等于
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  well, let me write T^2 / r^3,
  还是写成 T^2 / r^3 吧