When the magnet drops through the copper pipe, the moving magnetic field induces a circumferential current in the conducting pipe, which in turn gives rise to an induced magnetic field. This is predicted by Faraday's law, as stated on page 479. This induced magnetic field opposes the field that produced it. The magnet falls much more slowly than it would through a nonconducting pipe.
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If you are interested, the term to look up is "eddy currents".