From: tjroberts137@sbcglobal.net   
      
   On 1/13/19 4:23 PM, Luigi Fortunati wrote:   
   > Circular motion without centripetal force is impossible. And then we   
   > must find a plausible explanation for this case. There is a rotating   
   > table and a disc without friction. If a person in the laboratory   
   > gently lays the disk on the rotating table and if the friction is   
   > (really) 100% absent, the disk, in the absence of friction, can not   
   > be dragged from the table, so it remains still while the table turns   
   > downstairs without disturbing.   
      
   Note the disk remains "still" relative to the inertial frame in which   
   the center of the rotating table is at rest.In discussions like this you   
   must always mention which frame or coordiantes you are using.   
      
   > For the observer who rotates with the table, the disk travels around   
   > a circle without ever moving away or nearing the center of rotation.   
   >   
   > How can we justify the rotary motion in the accelerated reference if   
   > no centripetal force acts on the disc?   
      
   In the rotating frame there must be a centripetal force: the disk is   
   not "fleeing away from the center", so there must be a centripetal force   
   to balance the (fictitious) "centrifugal force" in the rotating frame.   
   Note this centripetal force is every bit as fictitious as the   
   "centrifugal force" -- both are artifacts of using non-inertial coordinates.   
      
   	In GR such fictitious "forces" are easy to identify:   
   	they are components of the connection, and are explicitly   
   	coordinate dependent. Remember that coordinates are   
   	arbitrary human constructs, which nature does not use, so   
   	quantities dependent on coordinates cannot possibly be   
   	part of a valid model of nature. Hence we call such   
   	forces "fictitious".   
      
   Stated differently: the "rotary motion" applies to the rotating table   
   and observer, not the disk.   
      
   Tom Roberts   
      
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