Aristotle brought together all the knowledge of the time and fused it into a coherent explanation of the universe. His system of physics was so tightly woven and comprehensive that it dominated European thought for nearly 2,000 years, despite that a great deal of what he taught was just plain wrong!
Aristotle's physics included the division of all matter into four elements, his laws of motion, the immobility of the Earth, his explanation of the heavens and the motion of the planets.
The Four Elements
Aristotle's Earth is made up of four terrestrial elements: fire, air, water, and earth. Each of these four elements has a position in the universe that it strives for. Fire strives to go to the heavens. Air strives to be in the atmosphere, and water and earth strive toward the center of the universe, which is the center of the Earth.
In Aristotle's physics, there are two type of motion: natural and forced. These two motions are never mixed; an object has one motion or the other. The natural motion for objects on Earth helps them to seek their proper location. The direction of movement is either up or down in accordance with their proper position in the universe.
According to Aristotle, the speed of a falling object is proportional to the weight of the object. Objects gain speed as they fall because they are getting nearer their destination; much like a horse increases its gate in anticipation of the stable.
Forced motion only continues to move an object while it is being pushed. When that force stops pushing the object, it stops moving. A special explanation is required to explain why a ball continues to move after it's tossed. The air is pushed out of the way to make way for the ball and then slips around behind to push it along. But eventually, the air gets "tired", and the ball returns to natural motion.
The Immobile Earth
The Earth is at the center of the universe and the center of the Earth is the absolute center. It was obvious to Aristotle that the Earth is not moving - otherwise we would feel the motion. It was equally obvious that the Earth must be perfectly unique. If there were two Earths, they would both be attracted to the same center of the universe, and they would join together.
In Aristotle's universe, there is a clear delineation between celestial objects, which are perfect and unchanging, and terrestrial objects, which are imperfect and changing. Celestial objects had their own set of rules. There are made of the fifth element, the aether, which permeates the entire universe.
The rules for motion are different for the fifth element. Since most of the stars rotate about the sky on a daily basis, it was natural to consider them as points of light on a huge crystalline sphere. The natural motion is circular for the fifth element because this motion is perfect.
The planets were a conundrum for Aristotle. Whereas most of the stars maintain their relationship to each other night after night, the planets usually move eastward with respect to the other stars, but occasionally move westward. To account for this wandering, each of the planets has its own crystalline sphere to transport it independently. Thus, a system of nested spheres holds the planets, the Moon, and the Sun.
Since motion of the celestial bodies requires that something pushes, there is a "prime mover" in order to make the stars move. This prime mover exists outside of the universe and impels the outermost celestial sphere. This sphere in turn moves the next sphere inward, and so on.
Aristotle's physics were incredibly influential for thousands of years despite inaccuracies. His system was cohesive and comprehensive and effected how scientists thought about physics and the world around them for centuries.