来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  That is the force which I do not know in magnitude.
  我并不知道这个力的大小
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  But this is the acceleration of the one kilogram mass.
  但这是 1 千克物体具有的加速度
• 
  
  Then, I bring the elephant
  然后我再找来大象
• 
  
  and I pull the spring by the same amount
  用相同的力去拉弹簧
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  and I find the acceleration of the elephant
  大象就具有了加速度
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  and the denominator is obviously the mass of the elephant.
  很明显 分母就是大象的质量
• 
  
  The force is not known,
  力的大小未知
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  but it's the same force.
  但与前一个力大小相等
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  So, when I divide these two numbers,
  把这两个式子相除
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  I'm going to find a1 over aE is equal to mE over m,
  就会发现 a1 比 aE 等于 mE 比 m
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  which is the one kilogram mass.
  m 等于 1 千克
• 
  
  What we needed was some mechanism of exerting some fixed force.
  我们需要的是一些能够产生恒力的装置