From: fortunati.luigi@gmail.com   
      
   The spring AB is a body.   
      
   If I exert the force F on the end A, the spring accelerates according   
   to Newton's second law F=ma and contracts with respect to its length at   
   rest.   
      
   Why is there this contraction if there is no opposing force on the   
   other side of the spring?   
      
   Or perhaps, there is an opposing force?   
      
   Luigi Fortunati   
      
   [[Mod. note -- There are two possibilities:   
      
   If the spring is *massless* (obviously this is an idealization, but it's   
   a useful case for conceptual purposes), then the spring doesn't contract   
   (it just accelerates as a rigid body), since as you notes thereis no   
   opposing force on the other side of the spring.   
      
   If the spring has *nonzero mass*, then the inertia of the various parts   
   of the spring provides the opposing force.  To work this out in detail   
   we'd need to write out equations of motion (Newton's 2nd law + Hooke's law)   
   for the individual parts of the spring, then solve these equations.   
   -- jt]]   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)