Reigning AMA Motocross national champion Ricky Carmichael, considered by many to be the greatest off-road motorcycle rider in history, abruptly announced his retirement from competition Monday after completing a summer course in physics at Florida State University. 'I've had a great run in both professional motocross and Supercross, but the more I learn about kinetic energy, momentum, and ballistics, I'm beginning to think I've had a pretty good run of luck, too,' said Carmichael, whose instructors said seemed particularly interested in the effects that gravity and sudden deceleration could have on a Suzuki RMZ250 four-stroke dirt bike. 'I'd like to thank everyone and everything that helped me get this far, especially the considerable gyroscopic forces of the wheels on my race bikes, which were apparently sometimes the only thing keeping me from sublimating into a liquid state of matter.' Carmichael is rumored to be considering a career in NASCAR after completing courses in business and marketing. 