What the Middle-Aged Galileo Told the Elderly Galileo: Galileo’s Search for the Laws of Fall

Abstract

Recent historiographic results in Galilean studies disclose the use of proportions, graphical representation of the kinematic variables (distance, time, speed), and the medieval double distance rule in Galileo’s reasoning; these have been characterized as Galileo’s “tools for thinking.” We assess the import of these “tools” in Galileo’s reasoning leading to the laws of fall ( $$v^{2} \propto D$$ and $$v \propto t$$ ). To this effect, a reconstruction of folio 152r shows that Galileo built proportions involving distance, time, and speed in uniform motions, and applied to them the double distance rule to obtain uniformly accelerated motions; the folio indicates that he tried to fit proportions in a graph. Analogously, an argument in Two New Sciences to the effect that an earlier proof of the law of fall started from an incorrect hypothesis (v ∝ D) can be recast in the language of proportions, using only the proof that v ∝ t and the hypothesis.