From: poutnik4REMOVEnntp@CAPITALSgmail.com   
      
   Luigi Fortunati  Wrote in message:   
   > 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?   
   >   
      
   Have you considered the option you are mistaken ?   
      
   The mass starts to push the string and vice versa initially with   
    zero force,   
   allowing for the mass full 1 g acceleration.   
      
   As the spring is contracting both acting and reacting forces increase   
   and the mass acceleration decreases.   
      
   As the contraction continues,   
   the string force is greater than m.g and the direction of   
    acceleration and then of motion is reverted.   
      
   If there is enough motion damping(?),   
   the position is quickly  stabilized at force  equilibrium.   
    Otherwise, there are temporary oscillations about the final position.   
      
   --   
   Libor Striz aka Poutnik ( a pilgrim/wanderer/wayfarer)   
      
      
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