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# Re: Bar magnets

I too had this problem. Instead of measuring the attraction, use a weaker
spring and use the repulsion of the magnets to shorten the spring. You
have to be careful about sideways motion when you get close.

Have you tried to measure the force by connecting the fixed magnet to a
'Force Probe' connected to a CBL or Computer? I have had great luck this
way and the pendulum motion that you see in a magnet suspended from a
spring is no longer a problem.

As far as why the attraction seems instant, I am not sure of the details
but, it seems to be a positive feedback situation. Once the magnet starts
moving, the force increases which decreases the distance which increases
the force which decreases the..... You get the idea.

PHYS-L@lists.nau.edu writes:
Without thinking too much I attempted an experiment
which failed, as described below. Comments and
suggestions will be appreciated.

I wanted to measure the attractive force between the
poles of two bar magnets (each 13 cm long and 1 cm
in diameter. Keeping these magnets by hand I started
to feel the attraction when the distance between the
coaxially positioned bars was 1 cm or so.)

I suspended one magnet from a brass spring whose
k was 10 N/m. The mass of the spring was 175 g
while the mass of the magnet was 61 g. Naturally,
the spring stretched a little. Then holding another
magnet by hand I started approaching its pole toward
the suspended magnet from below. I expected to see
additional stretching of the spring for each distance
between the poles, for example 1 cm, 0.75 cm and
0.5 cm and 0.25 cm. But this did not happen.

The suspended magnet moved suddenly to the hand-
held magnet to establish the contact (when the distance
between the poles was less than about 1 cm). This
happened regardless of how careful I was in trying
to prevent natural oscillations. What is wrong with
this attempt to measure the magnetic force Fm?

I expected the equilibrium at: k*x=mg+Fm.The
Fm increases when the distance between the poles
becomes smaller and I expected larger x for smaller
distances between the poles. What should I do?
I know that Coulomb used a torsion spring to
establish the relation between Fm and the distance
between the two poles. Why is my linear spring
different? Why was my equilibrium unstable?
Ludwik Kowalski

P. Lohstreter "If we want to solve a problem
that
US Science we have never solved before,
Chemistry & Physics we must leave the door to the
unknown ajar."
214-360-6389
Richard Feynman