From: julio@diegidio.name   
      
   On Saturday, 7 May 2022 at 12:38:42 UTC+2, Luigi Fortunati wrote:   
      
   > > A free-falling brick is an inertial frame?   
      
   A free-falling brick is *in* "an" inertial frame.  Which more precisely   
   means that we can find an inertial frame in which the brick is in   
   uniform motion, then just a special case is an inertial frame in   
   which the brick is at rest, and and even more special case is the   
   frame in which the brick is at rest at the origin of space.  And we   
   might call that last one "the brick's own frame", because there is   
   indeed something "privileged" about it as far as that brick is   
   concerned: OTOH, though, notice that the fact that the brick is in   
   *free-fall* requires no frame to state or verify at all, it's altogether   
   a *true* physical property that can be verified *locally*.   
      
   > > Are a pair of free-falling half-bricks an inertial frame or are they   
   > > two distinct inertial frame?   
      
   It should now be clear that that is simply upside down: if two   
   bricks are in free fall, of course one can find an inertial frame in   
   which both are in uniform motion, in fact infinitely many of them.   
      
   Julio   
      
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