A classroom demonstration, a variation of the simple swinging pendulum, is described. In our "portulum," a ball, driven by short blasts of air, rolls along a curved tube. The design of this device, its construction, and its usefulness to the teaching of physics are discussed. It is also shown that the oscillations of the rolling ball have the same mathematical form as the oscillations of the ball swinging along the same path, but with a lower frequency.
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Marschall, Laurence A. "Driven 'Portulum': A Rolling Ball as a Simple Oscillating System." American Journal of Physics (1981) 49(6):557-561.
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Original version available from the publisher at: http://ajp.aapt.org/resource/1/ajpias/v49/i6/p557_s1?isAuthorized=no