Magnetic braking

This apparatus lets an aluminium wedge swing freely between the poles of a strong magnet. We would not expect aluminium to attract to a magnet. It is like any pendulum and swings for a long time before friction dissipates its energy and it stops.

This aluminium wedge swings once or twice before stopping. What's the difference? Both wedges cut through the flux lines that go between the north pole and the south pole of the magnet. By Faraday's Law an EMF is induced that is proportional to the rate of flux cutting. The EMF causes circular currents to flow, which are called EDDY CURRENTS and which I have drawn onto the aluminium. They act like electromagnets and are repelled by the magnetic poles as they swing in, slowing the motion. They are attracted as they come out, also slowing the motion. It doesn't work on the first pendulum because the slots cut into that pendulum reduce the size of the eddy currents. The "repelling in and attracting out" is a consequence of Lenz's Law which is a magnetic version of the law that says you cannot create more energy. The eddy current electromagnets cannot push to make it go faster as that would be creating more energy.