PURPOSE: To show the transformation of a circular motion into a linear motion.
DESCRIPTION: A turntable rotates at 33 rpm. Mounted on the edge of the turntable platter is a ping pong ball. Above the platter is a simple pendulum with a period that exactly matches the period of the turntable and with a bob that is similar to the ping pong ball. The shadow projection of both the linear pendulum motion and the circular turntable motion is seen on the overhead screen projected by the carbon arc lamp.
References:
AJP 66, 10. October 1998, p. 927 "Comment on a ball rolling on a freely spinning turntable", by Warren Weckesser.
EQUIPMENT: Turntable and ball, simple pendulum, arc lamp, portable screen.
SETUP NOTES: This is a long set up. The day before this is performed a pendulum length of 78 cm is fine tuned to match the 33 rpm. by carefully adjusting the string length to exactness. Also, the arc lamp must be angled carefully to put the image onto the portable screen. Practice the synchronized release of the pendulum and warm up the arc lamp before class so it will refire quickly.
Updated by Jun Qi in 3/1/2000