From: bergervo@iae.nl   
      
   On 1/14/2019 7:11 PM, Sylvia Else wrote:   
   > On 14/01/2019 9:23 am, 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.   
   >>   
   >> 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?   
   >   
   > You misstated the rule. It should be:   
   >   
   > Circular motion without centripetal force is impossible in an inertial   
   > frame.   
   >   
   > An accelerated frame is not inertial.   
      
   Indeed it is common to restrict the rule to inertial frames.   
      
   One can, nevertheless, also extend it to more general cases   
   like this. Luigi should then accept that there is a Coriolis   
   force pointing exactly to the center with the right magnitude.   
   So there is a centripetal force, exactly the force needed.   
      
   --   
   Jos   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)