a thin, flat, vertical tank of water spins around on a turntable,
the water surface forms a parabola. The visitor turns a crank
which makes the turntable rotate, and thus controls the speed
of rotation. When the tank rotates rapidly the surface forms a
deeply curved parabola; when it rotates slowly it forms a shallow
parabola. The visitor can also use a magnet to position several
rods, which float vertically when the water is at rest. When the
tank rotates however, these rods align themselves perpendicular
to the surfacethey are no longer vertical.
The surface of the water is perpendicular to the net force in
the rotating frame of reference.