Magnetic Free Fall
There are two cylindrical slugs of metal were allowed to free-fall down a
hollow vertical tube made of copper. One of the slugs, a magnet, took
significantly longer to fall through the tube than the other.
The moving magnetic field induces an electric current in the surrounding
conductor, and the magnetic field resulting from this "eddy current"
opposes the field of the falling magnet. If the conductor were a
superconductor, that is, if no energy were lost by the induced current, the
induced magnetic field would be able to completely prevent the permanent
magnet from moving. This is the principle behind magnetic levitation. You
question sounds like a homework question; for the actual equations
governing this behavior, look in your physics book under "eddy currents" or
Richard Barrans Jr., Ph.D.
The changing magnetic flux sets up eddy currents in the copper which
produce their own magnetic field opposing the change in magnetic flux.
This produces a force on the magnetic slug.
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Update: June 2012