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Re: Bar magnets


I have created an IP simulation that seems to show all the
behaviors you have described and put it on my IP web site at


As I would expect, weak springs show the instability more clearly
than stiff springs. When the free dipole is raised toward the
lower end of the hanging dipole, the spring stretches to a new
equilibrium position. If the separation becomes too small, the
equilibrium becomes unstable and the upper dipole moves rapidly
downward until it contacts the lower dipole. After that happens,
lowering the free dipole again stretches the spring even more and,
eventually, the upper dipole is pulled back up. A hysteresis loop
is created in a plot of spring stretch as a function of lower
dipole position.

Anyway, check it out if you like.

John Mallinckrodt
Cal Poly Pomona

On Sun, 18 Feb 2001, Ludwik Kowalski wrote:

I replaced the attractive magnetic poles by electrified pith balls
(sphere A and sphere B) and performed an Interactive Physics
simulation (see the details below). The result is that the two
pith balls had no trouble of finding the equilibrium position;
they did not jump to each other suddenly, as my bar magnets

Does it mean that Interactive Physics is not good for this kind
of simulation or that the experiment was not good enough to
prevent small sidewise and vertical oscillations?

Here are the details (there is nothing special about them)

1) Spring constant k=30 N/m, spring natural length 1.4 meters.
2) Mass of the suspended sphere A was 0.5 kg, its charge was
+5*10^-5 C. In the absence of the second sphere B the initial
location of A was x=0, y=+1 meters.
3) The second sphere B was kept at a fixed position (x=0, y=-1m).
The mass of B was 0.5 kg, its charge was -5*10^-5 C.
4) Strong damping was imposed by setting air resistance at
"high" and by imposing 50 kg/m^2.
5) The electrostatic forces were turned on without turning off
6) As soon as the RUN button is pressed the sphere A starts
moving toward the sphere B, performs several oscillations
and remains at rest at y=0.506 meters.
Ludwik Kowalski