We report on a new optical technique that uses light forces and a time-of-flight spectrometer to make absolute measurements of cluster polarizabilities. This is also the first accurate report of an ac polarizability measurement of a condensable cluster in the gas phase. We have determined the optical polarizability of C-60 at the fundamental wavelength of a Nd:YAG laser (1.064 mu m) to be alpha=79 +/- 4 Angstrom(3).