Postprint version. Published in American Journal of Physics, Volume 71, Issue 5, May 1, 2003, pages 510-511.
Copyright © 2003 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 and may be found at http://dx.doi.org/10.1119/1.1564812.