where V=_A is taken as the characteristic velocity for a
given swing, A is the projected area of the sphere, _Rs
2, and
F is the average force over one period. Our photographs
allowed us to determine the height y of the sphere above the
lowest point at the center of the arc as well as the amplitude
A. The change in potential energy U can be easily related to
B. FIG. 1. Photographs of _a_ laminar flow at small amplitude oscillation
and _b_ a street of vortex rings at larger amplitude
oscillation.