This article is an in-depth study of Galileo's explanation and justification of the free fall law. It highlights the epistemological break with the Aristotelian Scholastic Tradition insomuch as it signifies a new scientific concept and a mathematical vision of the natural world. In a parallel way, a study in Spain of modern secondary school textbooks reveals that the explanation of the law shows a sequential order and mathematical resources similar to those used by Galileo. These similarities are analyzed from a didactic perspective. Other interesting features are also pointed out, such as those related to the use of proportionality and those related to the use and interpretation of graphics.