The article “Segmented Hoop as a Physical Pendulum” by William Layton and Nuria Rodriguez [Phys. Teach.51, 418 (Oct. 2013)] reminded me of a lab exercise that I used for many years in my AP Physics classes. We used wire hoops suspended from a knife edge with a small notch filed in the wire at the point of support, but the physics was the same. The use of the parallel-axis theorem is an excellent exercise for students, but there is also another way to derive the same result.

Imagine a massless hoop pivoted at the 12 o'clock position, with a pair of identical massive particles attached at symmetrical locations such as the 3 and 9 o'clock positions, or the 2 and 10 o'clock positions. The period of a physical pendulum is given by T2=(2π)2[I/(Mgs...

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