来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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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
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  has to be deduced every time with painstaking, experimental data.
  每次得到都不容易 需要实验的数据
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  Without that, F = m·a is useless.
  否则 F = m·a 没有意义
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  If you don't know what's on the left-hand side, independently--
  如果你不能独立地获知等式左边的数值
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  Let me repeat again,
  我再重复一遍
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  you don't say the force on the body is m·a.
  物体的受力不是因为你所说的 m·a
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  The force on the body is due to a spring,
  而是因为物体受到了弹簧
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  due to gravity, due to charges -- whatever.
  引力 或是电荷等等的作用力