来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  This will be the spring force transmitted from 1 to 2.
  这就是从 1 传到 2 的弹簧力
• 
  
  This will be the force of gravity,
  这就是重力
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  or it could be an electric force
  它也可能是电场力
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  or any other force due to anything else;
  或者别的什么力
• 
  
  we are not interested.
  这些我们不关心
• 
  
  Now, here is the interesting manipulation we're going to perform.
  现在我们将进行一个有趣的处理
• 
  
  We are going to add the two things on the left-hand side
  我们将把等号左边的这两项加起来
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  and equate them to whatever I get on the right-hand side.
  使其与等号右边相等
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  Then, I get m1·x1 double dot
  然后我就得到 m1·x1 两点
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  plus m2·x2 double dot,
  加上 m2·x2 两点
• 
  
  and that's going to be -
  结果等于
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  I'm going to write it in a particular way,
  我要把它写成这样