Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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



On Dec 21, 2007, at 11:37 AM, Bob Sciamanda wrote:

If you do a search (Google, Wikipedia, etc) you will find "dynamic
equilibrium" usefully defined
only for chemical and biological interactions. In Mechanics a
somewhat related set of useful concepts is stable vs unstable
equilibrium.

Note that in mechanics we say that an object at rest under the action
of a set of forces which sum to zero is in a state of static
equilibrium. This same object could be said to be in a state of
dynamic equilibrium when viewed from a different frame, in which the
object is in motion. This distinction does not add much and AFAICT is
not used.

Thanks Bob,
1) I changed the verbal description slightly at

http://pages.csam.montclair.edu/~kowalski/cf/339students.html

(see the ending of point 5).

2) Is there any reason to think that my electrical orbiting system (three equal masses) will be less durable that a binary star or a solar system? Unfortunately, the Interactive Physics does not allow to simulate in 3 dimensions.

3) Is my virtual displacements argument, and Figure 2, sufficiently convincing that the initial plane of the orbit will not change (in our laboratory frame)?

4) Using the IP simulation, I did what I wanted to do yesterday, but did not know how. The suggested idea was to create a short time disturbance and to show that original trajectories of rotating particles are not restored after the disturbed parameter is restored. This should have been obvious but a simulation was worth creating. The disturbed parameter was the mass of one positive particle. It was changed from 1.0 kg to 1.1 kg for the duration of 0.01 seconds. What do you think happened? Close your eyes, produce the answer and then read the next paragraph.

The particle whose mass was disturbed started moving away from the other two particles. These two particles started spiraling toward each other. They ended orbiting elliptically around each other (like a binary star, I suppose). The trajectory of the third particle was becoming a straight line. Naturally, I could have disturbed any other parameter, for example, the speed of one particle, the location of the negative particle, etc. The term "dynamic equilibrium" was not appropriate because it implied that the original trajectories would be restored (after restoring what was disturbed). The IP could be an effective teaching tool.
_______________________________________________________
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/