From: fortunati.luigi@gmail.com   
      
   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?   
      
   --   
   - Luigi Fortunati   
      
      
   [[Mod. note -- I think you're looking for the concept of an inertial   
   reference frame,   
     https://en.wikipedia.org/wiki/Inertial_reference_frame   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)