Sparking the Investigation of Naturally Accelerated Motion: The Pendulum Plane Experiment

Based on the experimental record preserved in the Notes on Motion, the chapter reconstructs and discusses an experiment which, thus far, has been almost completely overlooked. Galileo timed the swinging of a pendulum as well as the rolling of a ball down along a long, gently inclined plane. From the latter measured time, he theoretically inferred the time of motion along a shorter plane, which could be inscribed into the arc of pendulum swing as a chord. Galileo initially assumed that the swinging of the pendulum along a quarter arc would be completed in the same time as fall along a chord spanning this arc and had designed the experiment to test what is here referred to as the single chord hypothesis. The hypothesis had been suggested by a challenging similarity Galileo perceived to hold between pendulum motion and naturally accelerated motion along inclined planes. Yet it was not confirmed by the experiment. In consequence, Galileo altered his conceptualization of the relation between swinging and rolling, and thus, the experiment, conducted in all likelihood in 1602, came to mark the onset of a thoroughgoing investigation of this relation.