From: fortunati.luigi@gmail.com   
      
   Jonathan Thornburg [remove -animal to reply] mercoledì 11/04/2018 alle ore   
   20:09:17 ha scritto:   
   >> 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).   
      
   Ok.   
      
   >> 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).   
      
   If the mass does not exert any force on the spring and the spring does not   
   exert any force on the mass, nothing can move!   
      
   > 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).   
      
   How does the spring slow down the acceleration of the mass if the mass starts   
   at zero speed?   
      
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