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Two bar magnets of the same size are dropped through an aluminum tube and a glass tube.  The magnet dropped in the glass tube falls at the normal rate of acceleration due to gravity, but the magnet falling through the metal tube is slowed.

This slowed acceleration occurs because the falling magnet induces currents inside the metal tube.  The induced currents then produce a small magnetic field that opposes the direction of the original magnetic field.  This effect is known as

Two bar magnets of the same size are dropped through an aluminum tube and a glass tube.  The magnet dropped in the glass tube falls at the normal rate of acceleration due to gravity, but the magnet falling through the metal tube is slowed.

This slowed acceleration occurs because the falling magnet induces currents inside the metal tube.  The induced currents then produce a small magnetic field that opposes the direction of the original magnetic field.  This effect is known as Lenz's Law, a result of Faraday's law of induction.