From: helbig@asclothestro.multivax.de   
      
   In article , Luigi Fortunati   
    writes:   
      
   > The second principle says that F=m*a and F1=m1*a1. (1)   
      
   Yes.  I can call them what I want.  F_george=mass_george*a_george.   
      
   > The third principle says that for every action F corresponds an equal   
   > and opposite F1 reaction, ie an F1 such that F1=-F. (2)   
      
   Right.   
      
   > Substituting the (2) to (1) we have F=m*a and -F=m1*a1.   
      
   OK.   
      
   > Adding the first and second members we have: F-F=(m*a)+(m1*a1), from   
   > which the corollary: m*a=-(m1*a1) in which there are no forces but only   
   > equivalences between different masses with different accelerations.   
      
   You can't just add them.  The third principle says that for a specific,   
   given, F, there exists -F.  You can't just add this to the general   
   definition.  It "works" only because you wrote "F1 such that F1=-F".  In   
   other words, you chose to call it F1.  Why?  I could say "F2 such that   
   F2=-F" which is just as valid since now we are considering only the   
   third principle, but then the addition trick doesn't work.   
      
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