The Physics of Aristotle versus
The Physics of Galileo

Aristotle taught that the substances making up the Earth were different from the substance making up the heavens. He also taught that dynamics (the branch of physics that deals with motion) was primarily determined by the nature of the substance that was moving.

The Dynamics of Aristotle

For example, stripped to its essentials, Aristotle believed that a stone fell to the ground because the stone and the ground were similar in substance (in terms of the 4 basic elements, they were mostly "earth"). Likewise, smoke rose away from the Earth because in terms of the 4 basic elements it was primarily air (and some fire), and therefore the smoke wished to be closer to air and further away from earth and water. By the same token, Aristotle held that the more perfect substance (the "quintessence") that made up the heavens had as its nature to execute perfect (that is, uniform circular) motion. He also believed that objects only moved as long as they were pushed. Thus, objects on the Earth stopped moving once applied forces were removed, and the heavenly spheres only moved because of the action of the Prime Mover, who continually applied the force to the outer spheres that turned the entire heavens. (A notorious problem for the Aristotelian view was why arrows shot from a bow continued to fly through the air after they had left the bow and the string was no longer applying force to them. Elaborate explanations were hatched; for example, it was proposed that the arrow creating a vacuum behind it into which air rushed and applied a force to the back of the arrow!)

Galileo vs. Aristotle

Thus, Aristotle believed that the laws governing the motion of the heavens were a different set of laws than those that governed motion on the earth. As we have seen, Galileo's concept of inertia was quite contrary to Aristotle's ideas of motion: in Galileo's dynamics the arrow (with very small frictional forces) continued to fly through the air because of the law of inertia, while a block of wood on a table stopped sliding once the applied force was removed because of frictional forces that Aristotle had failed to analyze correctly.

In addition, Galileo's extensive telescopic observations of the heavens made it more and more plausible that they were not made from a perfect, unchanging substance. In particular, Galileo's observational confirmation of the Copernican hypothesis suggested that the Earth was just another planet, so maybe it was made from the same material as the other planets.

Thus, the groundwork was laid by Galileo (and to a lesser extent by others like Kepler and Copernicus) to overthrow the physics of Aristotle, in addition to his astronomy. It fell to Isaac Newton to bring these threads together and to demonstrate that the laws that governed the heavens were the same laws that governed motion on the surface of the Earth.