来自《耶鲁大学开放课程：生物医学工程探索 第25集 第25集》

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  You see a metal ball that is attached to the femur,
  这个金属球装在股骨上
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  and this ball fits into this socket.
  两者相互契合
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  Now, if you just had metal on metal
  那么 如果是金属间直接接触
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  and you tried to move that metal ball inside that socket,
  你要直接移动球窝内的金属球
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  that doesn't work so well.
  恐怕不会很顺畅
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  Metal on metal, there's some friction between these two.
  金属和金属间必然有一定的摩擦
• 
  
  If this was a motor and you had metal on metal,
  如果这是个发动机 那么在金属机件间
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  you'd put oil in between there to lubricate.
  你会涂上润滑油来润滑
• 
  
  This is not possible in this situation.
  而在这种情况下加润滑油是行不通的
• 
  
  You put something else in there to lubricate
  必须加入其他东西来润滑
• 
  
  and the something else is this white material.
  而这"其他东西"就是这个白色的物质
• 
  
  It's a layer of polymer,
  这是一层高分子层