[Ether]

If you are trying to control position, you could drive a speed-reduction gearbox connected to an arm with weights on the end. Physically limit the arm travel so no-one gets hurt.

If you are trying to control speed, you could:

a) drive a speed-increasing gearbox connected to a fan (make sure the fan has a shield to protect errant fingers and contain blade failures)

or

b) drive a speed-reduction gearbox connected to the shaft of a second motor, and short the second motor's leads.


Wear safety glasses.