From: fortunati.luigi@gmail.com   
      
   Two equal and opposite forces cancel out if they are applied to the   
   same body, and they do NOT cancel out if they are applied to two   
   different bodies.   
      
   So, if a horse, harnessed to a cart that's too heavy, can't get it   
   moving despite all its efforts, it means that the horse's forward   
   force F1 is canceled out by the backward force F2 of the cart, exactly   
   as if the two forces F1 and F2 were acting on the same body and not on   
   two different bodies.   
      
   Does this happen because the horse and cart, when harnessed, are   
   effectively a single body?   
      
   Luigi Fortunati   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)