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Name: Denise A.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: Sunday, September 15, 2002


Question:
At Australia's Wonderland Theme Park there is a ride that elevates a carriage of 4 seats up a tower 67 meters high. The carriage 'free falls' until the braking system comes into play. Apparently, 6 meters above the ground there are metal fins attached to the bottom of the tower which "disrupt a magnetic field generated by pairs magnets on each seat". (The pairs of magnets on each seat are positioned with unlike poles facing and the tower fin comes between them). I cannot understand how the disruption of the magnetic field fins effectively causes a repulsion and hence the carriage to come to a stop. ( I am a junior science teacher and a biologist not a physicist!)


Replies:
This is a particularly beautiful application of Lenz's Law. A changing magnetic field produces an electric field, either when the field is moving relative to you or you are moving relative to the field (Einstein thought about this most carefully, leading to his Special Theory of Relativity).

If a conductor is moved through a changing magnetic field, the resulting electric field will cause currents to be produced in the conductor. Then, as for all currents in magnetic fields, there will be a magnetic force on the conductor carrying the current.

Lenz's Law says that a force will be in a direction opposing the change producing it. Since, in your example, the change is caused by the magnets going downward, Lenz says the resulting force will be upward, thereby stopping the carriage or at least slowing it.

Notice that if Lenz's Law were backwards, perpetual motion and free energy would be present everywhere. Because if the force were in a direction AIDING the change that produced it, the change would be aided and increased, thereby producing an even larger force trying to increase the change, and so on. The universe would instantly be blown up! Luckily Lenz's Law is correct!

Richard J. Plano



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