From: fortunati.luigi@gmail.com   
      
   A spring, subject to a single force, accelerates in the direction of   
   force but does not compress (F = ma).   
      
   Instead a spring subjected to two equal and opposing forces is   
   compressed.   
      
   So just look at a spring (or any body) to understand how many forces   
   are acting.   
      
   If there is compression, the forces are two, if there is not, the force   
   is only one.   
      
   On the rotor of the movie   
   https://www.youtube.com/watch?v=m4EElvJBCiM   
   you can clearly see that people are compressed against the wall, so the   
   forces are two and not just one.   
      
   And how is it possible if the only real force acting on them is only   
   the centripetal?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)