In the August issue of this journal, Rezaeizadeh and Mameghani (hereafter R&M) analyze the motion of an aluminum pendulum bob swinging inside a vertically oriented parallel-plate capacitor, alternately being attracted to and repelled from each plate as it picks up positive and then negative charges.1 A similar demonstration uses two horizontal aluminum disks connected across a Van de Graaff generator. A ping-pong ball coated with silver paint is carefully tossed between the large-diameter circular plates. (The ball needs to enter nearly vertically so that it does not escape sideways, and with a large enough speed that it bounces back up to the top plate.)
In our apparatus, the two plates are held apart by a distance of x = 127 mm using a small-diameter plastic cylinder. (The lower plate similarly rests on a cylinder, so that both plates ring loudly when impacted by the bouncing ball.) The painted ball...
