From: fortunati.luigi@gmail.com   
      
   The second principle F=ma states that if we apply the net force F   
   to  the mass  of the wheeled cart, the cart accelerates.   
      
   And so far we all agree, I think.   
      
   Is the opposite also valid? In other words, is it correct to say   
   that  if NONE applies force to him, the trolley does NOT accelerate?   
      
      
   [[Mod. note --   
   Yes, provided that we're (making all the measurements) in an   
   inertial reference frame.  See   
     https://en.wikipedia.org/wiki/Inertial_reference_frame   
     https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion   
   for more details.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)