Further informationson the exhibit "Whirlwind brake"
If you let the magnet drop through the plastic tube, it falls downwards unbraked. It doesn't matter which magnet you use. With the aluminium tube on the other hand, the fall is slowed down. If you listen carefully, you can hear the magnet slide down the tube.
If you let the magnet fall through the tube, rotating magnetic fields are created in the tube. This change is called induction current. Put simply, the same pole is opposed to the magnet in the tube. If the magnet falls through the tube with the southern pole , the same pole is formed again below the magnet. However, as the same poles repel each other the fall of the magnet is slowed down.