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
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").
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
(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
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
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.