From: tjroberts137@sbcglobal.net   
      
   On 12/16/17 12/16/17   12:19 AM, Luigi Fortunati wrote:   
   > I'm in the car and, when cornering, the lighter on the dashboard, from   
   > my point of view, accelerates in a centrifugal direction.   
   >   
   > Since it is I who accelerate (together with the machine I am bound to)   
   > and not the lighter, can we say that my acceleration is "real" and that   
   > of the lighter is "apparent"?   
      
   This is just a question of what the words mean, or rather, what you want   
   them to mean.   
      
   "Centrifugal force" is purely an artifact of using rotating coordinates   
   (implicitly those of your car's interior), and is a fiction invented to   
   permit one to apply Newtonian mechanics in a rotating frame. As it is   
   coordinate dependent, it cannot possibly be "real" in the sense of   
   modeling some phenomenon in the physical world.   
      
   In Newtonian mechanics the concept "acceleration" can be fuzzy, because   
   you might use accelerated coordinates. In GR we have the concept "proper   
   acceleration" which is not fuzzy at all (it is independent of   
   coordinates) -- you and your car have nonzero proper acceleration, while   
   the lighter sliding on a frictionless dashboard has zero proper   
   acceleration [#].   
      
   	[#] Ignoring the upward force of dashboard on lighter and drag   
   	from air. These are not part of your question.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)