Re: [Phys-l] Sharing a problem for students

[Ludwik Kowalski <kowalskil@mail.montclair.edu>]

On Dec 20, 2007, at 5:25 PM, Ludwik Kowalski wrote:

> On Dec 20, 2007, at 11:34 AM, Ludwik Kowalski wrote:
>
> > A problem for students has just been posted at
> >
> > http://pages.csam.montclair.edu/~kowalski/cf/339students.html


I am having trouble with the concept of dynamic equilibrium. Consider a 
sun-planet system. Is the system in dynamic equilibrium when the

m*v^2/R=G*M*m/R^2

equation is satisfied (when the orbit is a circle around the sun at 
M>>m)? The following objection can be made. Change something to 
invalidate this equation, for example the speed v, and the orbit will 
change to an ellipse, or maybe to a hyperbola. The system will not 
return to the initial orbit (initial energy state). Each orbit is 
stable (ideally permanent). Is it appropriate to identify permanence 
with dynamic equilibrium?  I am asking because I used this term 
"dynamic equilibrium" (see the ending of point 5, in the URL above). 
What does this term mean and when should it be used? Perhaps saying 
"the orbit is stable" would be more appropriate. In any case, what I 
said would remain valid if one phrase is replaced by another. Do you 
agree? What is "dynamic equilibrium?"
_______________________________________________________
Ludwik Kowalski, a retired physicist
5 Horizon Road, apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at http://csam.montclair.edu/~kowalski/cf/