From: oliver.jennrich@gmx.net   
      
   Luigi Fortunati  writes:   
      
   > Phillip Helbigundress to reply a écrit   
   >>> It follows that the presence of an EXTERNAL force during the free fall   
   >>> in a gravitational field, contrasts with the inertiality described in   
   >>> the first principle.   
   >>>   
   >>> So I explicitly ask: is the free fall in a gravitational field inertial   
   >>> or accelerated?   
   >>   
   >> I'm not sure what your question is, but if you are thinking of the idea   
   >> that "a man falling from a roof feels no force", then this is true   
   >> actually only in the limit if infinitely small size.  An object of   
   >> finite size will feel tidal forces (and, of course, any   
   >> non-gravitational forces present such as surface tension).   
   >   
   > Is the object of finite size in free fall (in a gravitational field)   
   > inertial (that is, is it not undergoing any force) or is it accelerated   
   > (because it is subject to the EXTERNAL tidal forces)?   
      
   That is a pointless question. If the object is of finite size, it   
   doesn't make sense to ask if the *object* ist in free fall. Most parts   
   of the extended object experience an acceleration as the internal forces   
   (e.g. tension) prevent the respective volume element to move on a   
   geodesic.   
      
   Now, in practical terms, there are bodies (cf the test masses of LISA   
   Pathfinder) that are so small compared to the typical length scale of   
   the curvature that the tidal forces and the ensuing deformation of the   
   object is unmeasurable small. But fundamentally, almost all parts of an   
   extended body feel an acceleration as long as there is a mechanism to   
   exert internal forces.   
   >   
   > Second question: do the tidal EXTERNAL forces act only in a given   
   > reference system or do they always act?   
      
   They always act. You cannot "transform away" curvature.   
      
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