From: fortunati.luigi@gmail.com   
      
   Libor Striz giovedì 12/04/2018 alle ore 01:06:50 ha scritto:   
   >> 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.   
      
   How is it possible that the mass pushes the spring with zero force?   
      
   Can a spring be contracted without applying any force?   
      
   > As the spring is contracting both acting and reacting forces increase   
   > and the mass acceleration decreases.   
      
   How is it possible that the initial acceleration of the mass can   
   decrease if initially both the speed and the acceleration are equal to   
   zero?   
      
   When we "lean" the mass on the spring, they are both still and the   
   acceleration can only increase.   
      
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