From: richalivingston@gmail.com   
      
   On Tuesday, April 19, 2022 at 2:21:44 AM UTC-5, Luigi Fortunati wrote:   
   > In the lift stopped at the floor, the bodies accelerate both towards=20   
   > the floor and towards the center of the Earth (the two directions=20   
   > coincide).   
   >   
   > If the cables break and the elevator goes into free fall, the two=20   
   > gravitational accelerations no longer coincide.   
   >   
   > The bodies stop accelerating towards the floor but continue to=20   
   > accelerate towards the center of the Earth.   
   >   
   > Doesn't this mean that in the free-falling elevator the force of=20   
   > gravity has not disappeared at all but is well active?   
      
   It all depends on what frame of reference you are talking about.  For a   
   "stationary" frame there is a "force" of gravity causing free objects to   
   accelerate downwards.  Note however this this frame of reference is   
   not an inertial frame.  Per the Equivalence Principle, this frame is   
   equivalent to one in space that is accelerating "upward" at 1 g.   
      
   For a reference frame that is stationary wrt the falling bodies, there   
   is no force acting at all, everything is in free fall and "weightless".   
   This IS an inertial frame because in this frame if you release a free   
   body, it remains stationary wrt this frame.  There is no force acting   
   on it.  With respect to this inertial frame it is the earth that is   
   accelerating upward.  (I am ignoring tidal effects, which is a   
   second order effect.)   
      
   In the modern interpretation gravity is not a force, but what we   
   attribute to the force of gravity is really a non-Euclidean space-   
   time in the neighborhood of massive objects.  The only real force   
   is the one at your feet that is accelerating you upwards at 1 g.   
   When you are stationary on the surface of the earth you are not   
   in an inertial frame, but in one that is accelerating upwards at   
   1 g.  It is the free falling objects that are in an inertial frame and   
   not accelerating.   
      
   Rich L.   
      
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