curator dropped small, strong magnets though an "eddy tube,"
a thick copper pipe. The moving magnetic field sets up electrical
currents in the pipe, which in turn set up a magnetic field, which
-- doesn't it just figure? -- opposes the field of the permanent
magnets, making them slow down. Anyway, it's cool how slow they
And how slow is that?
As usual, it depends, on many things: the electrical conductivity
of the pipe, the strength of the permanent magnets, the geometry
of the everything, the mass of the falling thing, and so forth.
In this case, since we had two magnets, we decided to stick pennies
between them, thereby letting us vary the mass. Would more pennies
make the assembly fall more quickly?
It turned out that the times were short enough that stopwatch measurement
was impractical. (It might be with a longer, more expensive pipe.)
So we set up Vernier photogates at the top and bottom of the tube,
and did our best to drop the magnet sandwich from just above the
Pennies is the number of pennies between the magnets.
T1 is the time when the sandwich went through the
top photogate, in seconds.
T2 is the time when it went through the bottom
How does the time it took compare with what it would have taken
in free fall?
What seems to be the relationship between time
and number of pennies?
Clearly, forces are at work making the magnet sandwich fall so
slowly. How big are they? What do they depend on? Can you come up
with any models -- and distinguish between them using the data?
More information you might want: the two magnets had a combined
mass of 17.7g. And the average mass of the pennies we used was 2.66
grams (we had a few old ones in the set and were not careful about
which ones we used in any given drop).
(data by Tim Erickson, March 2004)
You can find some theory here.
Don't look until you have a theory of your own!