Date: Tue, 04 Oct 2011 10:25:48

Author: --- "Dale E. Stille"

Subject: Re: Air Track Collision "Discovery"



I would have thought that a horizontal mounting would be better as there
are significant eddy currents between the track and the magnet if the
magnet is too close. I'm thinking you will want to check the velocity
vs distance of the two arrangements. I know I was surprised at how much
the effect of eddy current damping was on the cart.


On 10/4/2011 9:31 AM, J. Terrence Klopcic wrote:
> Please ignore/excuse this "revelation" if it is already well-known.
> However, for those who, like me, hadn't known ...
> In our elastically-colliding-gliders-on-an-airtrack experiment, we use
> repelling magnets to avoid the losses associated with physical contact.
> The arrangement was to tape a little button magnet on each colliding
> glider with like poles facing outward. Although the collisions appeared
> smooth to the eye, we found that the calculated momentum and energy was
> not well conserved, especially when a smaller glider was pushed into a
> larger one. We surmised - and confirmed with high speed photography -
> that the opposing magnetic fields caused the gliders to tip slightly as
> one magnet is repelled to move above the other. Tipping, of course,
> destroys conservation since any tipping causes a jetting of the
> supporting air cushion and any touching of the glider to the track is
> fatal.
> This year, we introduced a remedy that seems to be resulting in far
> better momentum and energy conservation. Instead of using button magnets
> with opposing poles, we bought some small cylindrical magnets and
> mounted them with their poles vertical. Since the magnets on both
> gliders have their poles similarly aligned, they still repel, of course.
> However, if one considers the shape of the magnetic field from a bar
> magnet, one notices that the vertical gradient of the field is somewhat
> lower along the side of the bar than at the poles. We believe that this
> smaller gradient results in less tipping force, thus lessening the
> creation of outside forces on the glider system.
> Again, if this is old news to you, please ignore this e-mail. if you
> find it potentially useful, enjoy.
> Terry
> P.S. When taping anything to a glider, take care not to cover the air
> passage on the top of the "V". Doing so creates a little jet engine. :-[