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Aristotle on Motion | Describing Motion | Physics - Some two thousand years ago, Greek scientists understood some of the physics we understand today. They had a good grasp of the physics of floating objects and of some of the properties of light, but they were confused about motion.

One of the first to study motion seriously was Aristotle, the most outstanding philosopher-scientist in ancient Greece. Aristotle attempted to clarify motion by classification. 

He classified all motion into two kinds of motion: natural motion and violent motion. We shall briefly consider each, not as study material but as a background to modern ideas about motion.

In Aristotle’s view, natural motion proceeds from the “nature” of an object. He believed that all objects were some combination of four elements—earth, water, air, and fire and he asserted that motion depends on the particular combination of elements an object contains. He taught that every object in the universe has a proper place, which is determined by its “nature”; any object not in its proper place will “strive” to get there. For example, an unsupported lump of clay, being of the earth, properly falls to the ground; an unimpeded puff of smoke, being of the air, properly rises; a feather properly falls to the ground, but not as rapidly as a lump of clay, because it is a mixture of air and earth. Aristotle stated that heavier objects would strive harder and fall faster than lighter ones.

Natural motion was understood to be either straight up or straight down, as in the case of all things on Earth. Natural motion beyond Earth, such as the motion of celestial objects, was circular. Both the Sun and Moon continually circle the Earth in paths without beginning or end. Aristotle taught thatdifferent rules apply in the heavens and that celestial bodies are perfect spheres made of a perfect and unchanging substance, which he called quintessence.*

Violent motion, Aristotle’s other class of motion, is produced by pushes and pulls. Violent motion is imposed motion. A person pushing a cart or lifting a heavy boulder imposes motion, as does someone hurling a stone or winning a tug-of-war. The wind imposes motion on ships. Floodwaters impose it on boulders and tree trunks. Violent motion is externally caused and is imparted to objects, which move not of themselves, not by their nature, but because of impressed forcespushes or pulls.