From: tjoberts137@sbcglobal.net   
      
   On 3/12/23 3:55 PM, Luigi Fortunati wrote:   
   > Tom Roberts il 12/03/2023 12:23:34 ha scritto:   
   >>> An observer riding any particle of the string or sling stone   
   >>> feels the centripetal force of the innermost adjacent particle   
   >> Yes.   
   >>> and the centrifugal force of the outermost adjacent particle.   
   >> Nope.   
   >   
   > Nope? If you put yourself in the shoes of any particle of the string   
   > or stone, do you feel that the innermost adjacent particle pulls you   
   > towards the center and don't you feel that the outermost adjacent   
   > particle pulls you to the opposite side?   
      
   The adjacent particle on the outside exerts the centripetal force that   
   constrains the observer to move in a circle. There is no force exerted   
   by the adjacent particle on the inside, because the string is in tension   
   and is incapable of exerting an outward force.   
      
   Note also that "centrifugal force" is fictitious and appears only in the   
   rotating coordinates. It cannot ever be "felt" by an observer, because   
   it is not real (in any sensible sense of the word).   
      
   Remember that no coordinate-dependent quantity can model any real   
   phenomenon in the world we inhabit, because nature uses no coordinates.   
   (Coordinate dependence would mean that multiple calculated values would   
   all have to be equal to the single value of nature.) We humans use   
   coordinates to describe and model the world -- they are an arbitrary   
   construct of humans; coordinates are imaginary, though clocks and rulers   
   used to implement them are real.   
      
   >> "Centrifugal force" IS FICTITIOUS and appears in the rotating   
   >> coordinates...   
   >   
   > "Appears" in what sense?   
      
   In the sense that one must include it in order to apply Newton's laws in   
   those rotating coordinates.   
      
   > Can it be observed or measured?   
      
   No. One can measure the (centripetal) force of tension in the string by   
   placing a spring scale in the appropriate place. Such a scale cannot   
   measure the "centrifugal force" on an object because there is no place   
   to put its other end.   
      
   > Or is it just supposed to be there?   
      
   If one wants to calculate in the rotating coordinates, one must include   
   the "centrifugal force" -- otherwise Newton's laws to not describe what   
   happens.   
      
   Tom Roberts   
      
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