来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  You can replace the mass by a planet;
  你可以用一颗行星来替换这个物体
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  you can replace the spring by the Sun,
  用太阳替换弹簧
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  which is attracting the planet;
  它吸引着行星
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  you can put many planets,
  你可以放置许多行星
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  you can make it more and more complicated.
  可以让这个问题越来越复杂
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  But they all boil down to a similar situation.
  但他们都可以归结为同一个相似的情况
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  I know some information now
  现在已知一些条件
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  and I want to be able to say what'll happen next.
  我想就能解释接下来要发生什么
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  So, here I pull the mass,
  现在我拉这个物体
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  when I want A and I want to know what'll it do next.
  只要我知道 A 我就能知道接下来的情况
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  Remember, when we go back to the laws of Newton,
  大家回忆一下 当我们在讲牛顿定律的时候
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  the laws of Newton only tell you this --
  它只告诉了我们这些