CageLab: A low-cost apparatus to demonstrate rotational motion for advanced undergraduates

CageLab is an inexpensive, easily constructed apparatus for studying rotational motion that was designed as a central example for a unit in our junior-year curriculum. It consists of a rotor suspended so that the direction of its axis is either constrained or approximately free to move through an interesting range of orientations. The rotor’s simple construction from wire mesh (a rectangular cage), modeling clay (used to break the cage’s symmetry), and a detachable axle with simple bearings allows students to quantitatively compute its inertial properties starting from their own measurements of lengths and masses. The pendulum-like fixed suspension facilitates observations of the center-of-mass location and inertial moments. The trapeze-like mobile suspension allows observations of motion with spin along nearly arbitrary axes including stable, neutral, and unstable principal axes without using an expensive gimbal. Removing the clay leaves a symmetric rotor for which gravity-induced precession and nutation can be studied.