来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  Somebody had to stick his neck out and say,
  但必须有人愿意冒险提出定律
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  this is one of the first occasions.
  这就是其中的第一个
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  So now, you are going to apply this law to understand
  现在 你要通过应用这一定律来理解
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  what Kepler did.
  开普勒当时的想法
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  Okay, so what do you have to do to do what Kepler did?
  如何再现开普勒的推导过程呢
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  So, we are going to consider the motion
  我们就从一个围绕太阳运动的行星
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  of a planet around the Sun.
  开始考虑
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  So, let's put the Sun here;
  我们把太阳画在这里
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  let's put the planet here.
  把行星画在这里
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  The planet's at the location r.
  行星的位置矢量为 r
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  You go back, as usual, to the only equation in town: F = m·a.
  和平常一样运用唯一已知的公式 F = m·a
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  Now F, I told you,
  我告诉过你们 这里的F