From: fortunati.luigi@gmail.com   
      
   > [[Mod. note -- Assuming a person standing on (at rest with respect to)   
   >  the Earth's surface:  In Newtonian mechanics   
   > (a) Newtonian gravity exerts a downward force on the person, AND   
   > (b) The person's feet exert a downward force on the Earth's surface, AND   
   > (c) the Earth's surface exerts an upwards reactive force (reacting   
   >     against (b)) on the person's feet.   
   > The net vertical force acting on the person (=3D the sum of (a) and (c))   
   > is zero   
   > 	[(b) is not included in the sum because it's not acting   
   > 	on the person, but rather on the Earth's surface]   
   > , and hence the person has zero vertical acceleration with respect to   
   > the Earth's surface. =20   
   >   
   > In general relativity,   
   > (a) isn't there, AND   
   > (c) is still true, AND   
   > (b) is now categorized as a downwards reactive force on the Earth's   
   >     surface, reacting against (c).   
   > The net vertical force acting on the person is now just (c), and is   
   > upwards.  Thus the person accelerates upwards at 1 g acceleration   
   > relative to an inertial reference frame.  But in GR, an inertial   
   > reference frame is *free-falling*, so near the Earth's surface an   
   > inertial reference frame must have a 1 g accelreation downwards   
   > relative to the Earth's surface.  Thus the person's acceleration with   
   > respect to the Earth's surface is zero (=3D same as the Newtonian mechanics   
   > analysis).   
   > -- jt]]   
      
   You and Einstein say that the reference frames in free fall are   
   inertial.   
      
   Ok.   
      
   The elevator in free fall (relative to the Earth) is an inertial   
   reference frame.   
      
   And why is the Earth in free fall (relative to the elevator) NOT an   
   inertial reference frame?   
      
   Still, both of them are in free fall!   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)