Copyright 2009 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. This article appeared in American Journal of 
Physics 77 216 (2009) and may be found at
http://ajp.aapt.org/resource/1/ajpias/v77/i3/p216_s1?isAuthorized=no.