From: augustin.coppey@gmail.com   
      
   I believe the 3rd principle only works for static, solid and   
   non-elastic objects.  One could argue your examples with liquids   
   or moving objects, and find that 3rd principle fails, which is not   
   correct.   
      
   Separately, the definition of the reaction force of a spring is NOT   
   "an equal and opposite force on the mass". It is a force "proportional   
   (1st approximation) to the displacement of the spring".   
      
   [[Mod. note -- If you consider two bodies touching, the contact forces   
   between them are directly exerted on a very thin surface layer.  Because   
   that surface layer is so thin, compression waves can traverse it very   
   quickly, so it's a good approximation to treat it as non-elastic.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
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