Describe the physics behind how a magnet is slowed while falling through a copper tube. If a long slit were cut along the copper tube how would the magnet behave as it falls?

According to Lenz's law the magnet slows down.

Magnet drop without slowing down.

Explanation:

When the magnet is falling in the copper coil, then flux through the coil increases but according to Lenz's law in copper tube current will induce in that direction where the magnetic flux decreases. So, the magnet is repelled and slows down.

And when it is moving down after passing through a copper tube, then the flux decreases and the current will increases in reverse direction due to Lenz's law and will attract a magnet and again due to this it slows down.

If a long slit is cut along the copper then the induced current drops a lot and force also drop. So, the magnet passes through the coil in a shorter time or without slowing down.