Postprint version. Published in American Journal of Physics, Volume 61, Issue 9, September 1, 1993, pages 862-863.
Copyright © 1993 American Association of Physics Teachers. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Association of Physics Teachers. The following article appeared in American Journal of Physics.
The definitive version is available at http://dx.doi.org/10.1119/1.17427.