From: fortunati.luigi@gmail.com   
      
   The rigid elevator descends in free fall with the acceleration of its   
   mid point M.   
      
   In the elevator there is the ball A that is above M and the ball B   
   below.   
      
   Ball A acceleration is higher than the elevator acceleration and   
   therefore tends to approach the floor.   
      
   The acceleration of the ball B is less than the acceleration of the   
   elevator and therefore tends to approach the ceiling.   
      
   This is the tidal effect inside the elevator.   
      
   If it's all right here, I ask: Does the forces pushing (or seem   
   pushing) A to the floor and B to the ceiling are real or apparent?   
      
   --   
   Luigi Fortunati   
      
   Credere e' piu' facile che pensare   
   Believing is easier than thinking   
      
   [[Mod. note -- Note that you have described tidal forces which are   
   compressive in the vertical plane.  The actual tidal forces near the   
   Earth's surface are just the opposite, i.e., in an actual freely-   
   -falling elevator near the Earth's surface A would tend to approach   
   the elevator ceiling and B would tend to approach the elevator floor.   
      
   That said, the answer to your question depends on the context:   
   * In Newtonian mechanics, gravity -- including the tidal effects you   
     describe -- is viewed as a real force.   
   * In general relativity, gravity isn't viewed as a force; rather one   
     simply says that both balls A and B are free-falling, and the Riemann   
     curvature tensor describes the relative acceleration of nearby   
     free-falling bodies.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)