abstract: Among the scholars belonging to the so-called Galilean school, Giovanni Alfonso Borelli (1608-1679) shares with Evangelista Torricelli the widest interests of research. While most of Galileo’s followers specialize in one particular subject, Borelli's contributions cover almost every field of Galileo's research: mathematics, astronomy, mechanics to mention only his main interests. On the other hand, if the subjects are the same or at least very similar, when we look at the aim and method of research we cannot help but noticing important differences. In my talk, I shall examine in particular Borelli’s De vi percussionis, with the purpose of clarifying its content and method.