Reconstruction of Huygens’ gedanken experiment and measurements based on video analysis tools.

In this paper we describe the practical realization and the analysis of a thought experiment devised by Christiaan Huygens which was pivotal in his derivation of the formula for the radius of gyration of a compound pendulum. Measurements are realized by recording the experiment through a digital camera, and using a video analysis and modelling software tool to process and extract information from the acquired videos. Using this setup detailed quantitative comparisons between measurements and theoretical predictions can be carried out, focusing on many relevant topics in the undergraduate physics curriculum, such as the “radius of gyration”, conservation of energy, moment of inertia, constraint and reaction forces, and the behavior of the center of mass.