From: gerry@bindweed.com   
      
   In article , fortunati.luigi@gmail.com   
   says...   
   >   
   > Centrifugal force is an "apparent" force.   
   >   
   > Ok, but I have a doubt and I hope you can clarify it.   
   >   
   > The sling rotates and the hand exerts a real centripetal force on the   
   > rope.   
   >   
   > For the third principle, even the rope exerts an equal and opposite   
   > force (hence centrifuge) on the hand.   
   >   
   > How is possible that the two opposing forces, one is real and the other   
   > apparent?   
      
   The adjectives generally used to describe those forces are "pseudo" or   
   "fictitious", though perhaps "apparent" would be as good or better.   
      
   I prefer "pseudo" to "fictitious" precisely because reality is what   
   exists, and is therefore a poor criterion for separating our   
   categorisations of it.   
      
   In a rotating reference frame (which is something invented by us, not   
   part of nature), centrifugal force exists.  It causes objects to move   
   away from the centre of rotation when they are not subject to any other   
   force.  This is a model made by us, perfectly consistent as far as it   
   goes.   
      
   We can also look at the situation in terms of an inertial reference   
   frame (also something invented by us).  Then we see the the centrifugal   
   force as the reaction to a centripetal force that is causing a body to   
   move non-inertially.   
      
   We generally consider this description as simpler.  More importantly,   
   though, it is far much more extensible to the universe at large.  That   
   is to say, a rotating reference frame may be a useful way of thinking   
   about a certain group of objects, but we shall probably have to abandon   
   it if we want to describe their interactions with other objects.  The   
   inertial framework does not have the same problem, or at least not to   
   anywhere near the same extent.  [If you are willing to think of   
   cosmology in Milne-like terms, it can go all the way.]   
      
   So, the inertial framework is more extensible, more general.  That is   
   why we think of it as more fundamental, and give epistemological   
   priority to forces described in its terms.   
      
   - Gerry Quinn   
      
   [[Mod. note -- Irrelevant ad for antivirus software snipped. -- jt]]   
      
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