In a paper (posthumously) co-authored by Isaac Newton himself, the primacy of geometric notions in pedagogical expositions of centripetal acceleration has been clearly asserted. In the present paper we demonstrate how this pedagogical prerogative can inform the design of an experiment involving an accelerometer-equipped smartphone rotating uniformly in a horizontal plane. Specifically, the location of the sensor itself within the body of the smartphone will be determined using a technique that is purely geometrical in nature, relying on nothing more than the notion that centripetal accelerations are centrally pointing. The complete absence of algebraic manipulations obliges students to focus exclusively on the development of their geometrical reasoning abilities. In particular, it provides a healthy challenge for those algebraically accomplished students for whom equations, calculations, and data tables represent a means of avoiding a direct confrontation with the imposing spectre of material that is otherwise purely conceptual in nature.

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Sylvania STT102USB Portable USB Encoding Turntable Record Player in Suitcase, currently available online for under $100 US.
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The apparatus has been baptized as the “SpinFrame.” The 3D files and assembly instructions have been released under a CreativeCommons (CC-BY-NC) license and are available from the author upon request.
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Among the many apps available for this purpose, we chose Physics Toolbox Accelerometer for Android devices and Accelerometer ( by “Dream Arc”) for iOS devices.
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We found an angular rate of 78 rpm to provide data with a good signal-to-noise ratio.
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We have designed a set of pre-lab exercises that lead students towards this type of reasoning without, in our opinion, being overly prescriptive. They have been released under a CreativeCommons (CC) license and are available from the author upon request.
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