From: richalivingston@gmail.com   
      
   On Tuesday, April 26, 2022 at 2:21:54 PM UTC-5, Luigi Fortunati wrote:   
   ...   
   > 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!   
      
   You need to be more precise about what frame you are talking about.   
   The center of gravity of the earth is in free fall (around the sun), but   
   a reference frame tied to the surface of the earth is not, due to the   
   distortion of space-time by the mass of the earth.  An elevator on   
   one side of the earth in free fall is an inertial frame, but an elevator   
   in free fall on the opposite side of the earth is a different inertial   
   frame.  Each of these inertial frames will see the other as accelerating.   
   That doesn't mean either of these frame are not inertial.  The property   
   of being an inertial frame is a local thing.   
      
   The earth is not "in free fall relative to the elevator".  Relative to the   
   elevator the earth is accelerating upwards.  It should not be   
   considered an inertial frame because in that frame it is accelerating.   
   That is, an object in the surface-of-the-earth frame can only be   
   stationary in that frame if it has a force accelerating it upwards.   
      
   Rich L.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)