One of the canonical, and memorable, classroom demonstrations from an upper-division mechanics course is to toss a rigid body with three distinct principal moments of inertia into the air, giving it a spin along one of its three principal axes. A student’s mechanics textbook itself works great for the body, secured rigidly shut with a rubber band. The book will spin stably about its longest and shortest dimensions, just like a top or gyroscope. What is surprising is that any attempt to spin the book about its intermediate axis (the axis parallel to the book’s lines of text) will result in a wildly unstable and chaotic tumbling, which most students find curious enough to warrant staying awake for a subsequent derivation of Euler’s equations. However, now that most students read their text “books” off of a tablet or phone, this demonstration may seem outdated. Or is it? Like a textbook, a phone or tablet also has three distinct principal moments. Better still, not only do these solid state devices require no rubber band, but these bodies can collect detailed data on their dynamical state, turning a demonstration into an actual experiment. This article reports the results of this modern version of the classical “book toss” demonstration, fittingly carried out by a team composed of both an old physics professor and a youthful group of three undergraduate students studying physics and engineering.

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Note that in this experiment there is no need to correct, or tare, the raw rotational accelerometer data to remove the acceleration due to gravity. By the equivalence principle, the phone experiences no gravitational inertial effects during this experiment; since it is in free fall the results are the same as if we’d performed the experiment in empty outer space. This is fortuitous because if the phone was sensitive to the surrounding gravity, this local gravitational acceleration would be rapidly spread and vary across all three x, y, z phone axes, and be difficult to untangle and remove from the desired acceleration data.
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