from The STEMcell Science Shop
Regular price $2.00
A simple-looking pair of sticks holds some interesting physics. If you rub the plain stick along the ridges of the other, not much happens. But if you rub it while also letting a finger rub along the side of the ridges, the propeller spins. Switching the side of the ridges you finger rubs also switches the direction of rotation.
Rubbing the stick without touching the side creates a vertical alternating driving force, but touching either side introduces a horizontal force which causes rotation.
The principle behind this mechanism is similar to a hula hoop. The vibration with horizontal influence twirls the center hole of the propeller around its shaft.