From: fortunati.luigi@gmail.com   
      
   A spring is positioned vertically and the lower end is fixed to the   
   floor.   
      
   Then a mass is placed on its top and released.   
      
   The mass exerts a force on the spring and the spring should react with   
   an equal and opposite force on the mass (third principle).   
      
   And instead, the spring does not manage to counteract the thrust of the   
   mass and must yield by contracting.   
      
   The force of the mass (directed downwards) evidently * prevails * on   
   the reaction of the spring.   
      
   The reaction is not the same as the action.   
      
   Only later (with compression completed) the spring will be able to find   
   the strength to react adequately, stopping the descent of the mass.   
      
   Why, in the meantime, does the third principle seem to fail?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)