From: jthorn@astro.indiana-zebra.edu   
      
   Luigi Fortunati  wrote:   
   > A spring is positioned vertically and the lower end is fixed to the   
   > floor.   
      
   We are assuming the spring is in equilibrium at this point, i.e., the   
   spring is at its equilibrium length (taking into account its own weight).   
      
   >   
   > 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).   
      
   [[...]]   
      
   On the contrary, *at this time* (before the mass has moved and the   
   spring has compressed), the spring is (by assumption) still at its   
   equilibrium length and hence it exerts *no* force on the mass (and   
   hence by Newton's 3rd law the mass exerts no force on the spring).   
      
   Later, after the mass has moved downward and the spring is somewhat   
   compressed, the spring will indeed exert an upward force on the mass,   
   slowing the mass's downward acceleration (and the mass will exert a   
   downward force on the spring).  But you asked about the situation   
   at the initial time.   
      
   --   
   -- "Jonathan Thornburg [remove -animal to reply]"    
      Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA   
      currently visiting Max-Plack-Institute fuer Gravitationsphysik   
                         (Albert-Einstein-Institut), Potsdam-Golm, Germany   
      "There was of course no way of knowing whether you were being watched   
       at any given moment.  How often, or on what system, the Thought Police   
       plugged in on any individual wire was guesswork.  It was even conceivable   
       that they watched everybody all the time."  -- George Orwell, "1984"   
      
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