From: carlip@physics.ucdavis.edu   
      
   On 7/2/19 4:36 AM, Luigi Fortunati wrote:   
      
   [...]   
   > 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)?   
      
   If the object is held together by internal forces, then most bits of   
   the object aren't inertial -- they're subject to the internal forces.   
   If you work hard enough, you can define a center of mass position   
   that moves inertially.  That seems obvious, and I can't believe   
   that's what you're really trying to ask.   
      
   So let's take an example that avoids this, a ball of "dust," particles   
   that aren't bound to each other.  If the ball starts out spherical,   
   it will be distorted in roughly the same way your gelatin sphere is   
   (not exactly the same way, of course, because there are no longer   
   any internal forces).  In this case, each particle is inertial; the   
   shape changes because inertial motion is different at different   
   locations.  The distortion is called "geodesic deviation" -- look   
   it up if you want details.   
      
   > Second question: do the tidal EXTERNAL forces act only in a given   
   > reference system or do they always act?   
      
   Tidal "forces" in general relativity are proportional to the   
   curvature tensor.  If they are nonzero in any reference frame,   
   they're nonzero in all frames.   
      
   Steve Carlip   
      
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