From: nospam@de-ster.demon.nl   
      
   Luigi Fortunati  wrote:   
      
   > The second principle (F=ma) states that, if we apply a force F to the   
   > body of mass , it accelerates by a proportional quantity .   
   >   
   > What acceleration is it?   
   >   
   > Is it an acceleration with respect to anyone or with respect to a   
   > specific reference system?   
      
   The proper relativistic generalisation of Newton's F=ma is the 4-vector   
   equation  F=ma, with F the 4-vector force, m the scalar mass, aka the   
   rest mass, and a the 4-vector acceleration.   
      
   Since it is a 4-vector equation it is coordinate independent,   
      
   Jan   
      
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