Answer
See answers. This is an eddy current effect.
Work Step by Step
Consider the tube to be made of stacked metal rings. As the bar magnet falls, the magnetic flux in the tube sections right below it is increasing, while the magnetic flux in the tube sections right above it is decreasing,
By Faraday’s Law, the changing flux induces a current to flow around the circumference of the tube walls, which in turn creates an opposing magnetic field (Lenz’s law) that will slow the motion of the falling magnet.
The faster the magnet is falling, the faster the change in flux, and the larger the opposing magnetic force. Eventually, the magnet reaches terminal velocity, where the weight, mg, is balanced by the upward force caused by the eddy currents.
Note: the magnet never actually stops, since in that case the flux would be constant and there would be no induced current and no braking magnetic field.
