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Re: [Phys-l] Simulating a disturbance of a stable planetary system.


You seem to be repeating the same thing over and over expecting hoping to get different results.

Personally, I don't trust Interactive Physics - I prefer to program a problem in Fortran or C. However, try using the random number generator in I.P. to produce a series of small disturbances at random times and random directions. A single disturbance has to change the orbit. Real orbits are subject to both random and periodic disturbances.

Bob at PC


From: on behalf of Ludwik Kowalski
Sent: Sun 12/30/2007 9:45 PM
To: Forum for Physics Educators
Subject: [Phys-l] Simulating a disturbance of a stable planetary system.

On Dec 30, 2007, at 7:16 PM, John Mallinckrodt wrote:

. . . An IP simulation will easily demonstrate this fact in a
minute or two and it will not be a computational artifact. . . .

That issue emerged from my failure to demonstrate stability of a simple
two body system (a single planet revolving the sun along a circular
trajectory). We expect such system to be stable (persistent). Using the
I.P. (Interactive Physics) I simulated the system and a short
disturbance. Someone wrote that stability means ability to recover
after a disturbance. In my simulation the new orbit (after the
disturbance) was significantly different from the orbit before the
disturbance. The period of revolution of the new (elliptical) orbit
turned out to be longer that period of revolution of the initial
(circular) orbit. In other words, the disturbance I applied was not

The idea was to show that a disturbance applied to a two-body system is
self-correcting while the same disturbance applied to the three-body
system is not self-correcting. How to implement an I.P. disturbance
whose consequences disappear after the disturbance is over? I changed
the subject line of the thread because this question has nearly nothing
to do with what has been discussed earlier today.
P.S. To trust results of an experiment one often tests instruments by
performing control experiments. The two-body simulation was to be a
control experiment before the three-body simulation. But I was stuck,
as described in a message posted two days ago.
Ludwik Kowalski, a retired physicist
5 Horizon Road, apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at
Forum for Physics Educators