From: julio@diegidio.name   
      
   On Friday, 15 July 2022 at 01:57:10 UTC+2, Luigi Fortunati wrote:   
   > Julio Di Egidio alle ore 07:29:41 di gioved=EC 14/07/2022 ha scritto:   
   > >   
   > > There is indeed a corresponding centripetal force in the inertial frame in   
   > > which the bucket rotates ...   
   >   
   > The centripetal force exerted by the walls of the bucket on the water   
   > is already present before the bucket starts to rotate, because it must   
   > counteract the centrifugal thrust of the water which, even when   
   > stationary, would be set in motion centrifugally outwards if the walls   
   > of the bucket did not oppose.   
      
   But that is not "Newton's bucket", it's just something else.  Moreover,   
   and more basically, centripetal/centrifugal is not just any old force,   
   in fact has not even to do with shapes and "containers", it is   
   specifically how we call forces that derive from *rotational motion*.   
      
      
   > This ratio between the centrifugal and centripetal forces changes when   
   > even the water starts to spin!   
      
   That just cannot be: those two forces are just two different descriptions   
   of the same physics, typically associated with the two distinct frames   
   of reference, the one inertial in which the thing (bucket, spinning top,   
   whatever) is rotating, and the one rotating with the thing.   
      
   In fact, more concretely, whichever the frame of reference we   
   choose, we can draw vectors representing both the centripetal and   
   the centrifugal force (as measured in their respective frames) and   
   those two vectors, unless I am badly mistaken, stay identically   
   equal and opposite.   
      
   Incidentally, this is not Newton's third law, though it looks analogous   
   since it's another case of equal and opposite: the two forces in the   
   third law actually both exist, are not simply two sides (descriptions)   
   of the same coin...   
      
   > Is it the centrifugal force that pushes the water to accumulate against   
   > the walls of the bucket or is it the centripetal force that pushes the   
   > walls of the bucket to tighten against the water?   
   >   
   > [[Mod. note -- It appears that you're confusing two quite different   
   > forces:   
      
   While I second what the moderator goes on explaining there,   
   I think that more basic and to the point here was to note that   
   centrifugal/centripetal are, as said, just two sides of the same   
   one coin.   
      
   Julio   
      
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