Linear Motion
More than 2000 years ago, the ancient Greek scientists were familiar with some of the ideas in physics that we study today. They had a very good understanding of some of the properties of light, but they were confused about motion. Probably the first to c study motion seriously was Aristotle, the most outstanding philosopher-scientist in ancient Greece. Aristotle attempted to clarify motion by classification.
Aristotle divided motion into two main classes: natural motion and violent motion. We shall briefly consider each, not as study material, but only as a background to present-day ideas about motion.
Natural motion was thought to proceed from the "nature" of objects. In Aristotle 's view, every object in the universe had a proper place, determined by this "nature"; any object not in its proper place would "strive" to get there. Being of the earth, an unsupported lump of clay properly fell to the ground; being of the air, an unimpeded puff of smoke properly rose; being a mixture of earth and air but predominantly earth, a feather properly fell to the ground but not as rapidly as a lump of clay. Heavier objects were expected to strive harder. Hence, objects were thought to fall at speeds proportional to their weights: the heavier the object, the faster it was thought to fall.
Natural motion could be either straight up or straight down, as in the case of all things on earth, or it could be circular, as in the case of celestial objects. Unlike up-and-down motion, circular motion was seen as being without beginning or end, repeating it- self without deviation. Aristotle believed that different rules applied in the heavens, and he asserted that celestial bodies were perfect spheres made of a perfect and unchanging substance, which he called ether. (The only celestial object with any detectable change or imperfection was the moon, which, being nearest the earth, was thought by medieval Christians to be somewhat contaminated by the corrupted earth.)
Greek philosopher, scientist, and educator Aristotle was the son of a physician who personally served the king of Macedonia. At 17 he entered the Academy of Plato, where he worked and studied for 20 years until Plato's death. He then became the tutor of young Alexander the Great. Eight years later he formed his own school. Aristotle's aim was to systematize existing knowledge, just as Euclid had systematized geometry. Aristotle made critical observations, collected specimens, and gathered together, summarized, and classified almost all existing knowledge of the physical world. His systematic approach became the method from which Western science later arose. After his death, his voluminous notebooks were preserved in caves near his home and were later sold to the library at Alexandria. Scholarly activity ceased in most of Europe through the Dark Ages, and the works of Aristotle were forgotten and lost. Some of the early Greeks' ideas, however, became incorporated in the scholarship that continued in the Byzantine and Islamic empires. Various texts were reintroduced to Europe during the eleventh and twelfth centuries and translated into Latin. The Church, the dominant political and cultural force in Western Europe, first prohibited the works of Aristotle and then accepted and incorporated them into Christian doctrine. Any attack on Aristotle was an attack on the Church itself.
Violent motion, Aristotle ' s other class of motion, resulted from pushing or pulling forces. Violent motion was imposed motion. A person pushing a cart or lifting a heavy weight imposed motion, as did someone hurling a stone or winning a tug-of-war. The wind imposed motion on ships. Floodwaters imposed it on boulders and tree trunks. The essential thing about violent motion was that it was externally caused and was imparted to objects; they moved not of themselves, but were pushed or pulled.
The concept of violent motion had its difficulties, for the pushes and pulls responsible for it were not always evident. For example, a bowstring moved an arrow until the arrow left the bow; after that, further explanation of the arrow's motion seemed to require some other pushing agent. Aristotle imagined, therefore, that a parting of the air by the moving arrow resulted in a squeezing effect on the rear of the arrow as the air rushed back to prevent a vacuum from forming. The arrow was propelled through the air as a bar of soap is propelled in the bathtub when you squeeze one end of it.
To sum up, Aristotle taught that all motions resulted either from the nature of the moving object or from a sustained push or pull. Provided that an object was in its proper place, it would not move unless subjected to a force. Except for celestial objects, the normal state was one of rest.
Aristotle's statements about motion were a beginning in scientific thought, and al- though he did not consider them to be the final words on the subject, his followers for nearly 2000 years regarded his views as beyond question. Implicit in the thinking of ancient, medieval, and early Renaissance times was the notion that the normal state of objects was one of rest. Since it was evident to most thinkers until the sixteenth century that the earth must be in its proper place, and since a force capable of moving the earth was inconceivable, it seemed quite clear that the earth did not move.
It was in this climate that the astronomer Copernicus formulated his theory of the moving earth. Copernicus reasoned from his astronomical observations that the earth traveled around the sun. For years he worked without making his thoughts public-for and the two reasons. The first was that he feared persecution; a theory so completely different from common opinion would surely be taken as an attack on established order. The second reason was that he had grave doubts about it himself; he could not reconcile the idea of a moving earth with the prevailing ideas of motion-the concept of intertia was unknown to him. Finally, in the last days of his life, at the urging of close friends he sent his De Revolutionibus to the printer. The first copy of his famous exposition reached him on the day he died - May 24, 1543.