Re: Bar magnets

[Pete Lohstreter <plohstreter@MAIL.HOCKADAY.ORG>]

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



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