Lenz’s law. A falling magnet changes the magnetic flux through the pipe. If the pipe is a conductor, currents (eddy currents) flow around it in just the direction that opposes the change. They are in the magnet’s own field, so they push back on the magnet, upwards, against the fall.

Braking force ∝ speed, so F = kv and the terminal speed is v = mg/k

Direction of the currents. With the S pole at the top: below the magnet the downward flux is growing, so the current makes an upward field, which is anticlockwise seen from above. Above the magnet the downward flux is shrinking, so the current makes a downward field, which is clockwise.

The three pipes. Plastic is an insulator: no eddy currents, so the magnet falls freely, with a = g. Copper (5.9×10⁷ S/m) and aluminium (about 3.5×10⁷ S/m) both brake the magnet, and copper does it more strongly, by a factor of about 1.7 for walls of the same thickness. Both magnets reach a steady terminal speed when the braking force equals the weight.

The size of the currents. The currents grow with the speed, so they start at zero and build up. At terminal speed the braking force has to equal the weight in both pipes, so the currents are the same size: the better conductor just gets there at a lower speed.

The numbers are in relative units, so that you can compare the pipes. The animation is slowed down compared with real life.