Re: Bar magnets

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

Ludwik,

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

 <http://www.csupomona.edu/~ajm/myweb/index.ip.html>

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      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm

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
> did.
>
> 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
>      gravity.
> 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