Aristotle divided motions into the sublunary and the superlunary spheres. In the superlunary sphere motions did not stop. They were kept going by the Prime Mover (abount which nothing else was said except it kept the stars, the planets, the moon and sun moving). The natural motion here was circles with uniform motion around the circle. (Natural motion is motion which does not need to be explained. It is what things do when nothing else causes anything else to happen). Motion in the sublunary sphere, ie, on the earth is different. Object seek they home where they want to be at rest. The four elements out of which everything in this sphere are made -- earth, water, fire, and air -- have natural homes. Earth and water want to be at rest on the on the earth. Fire and air naturally want to be at rest far away from the earth. Thus when not held away from the ear by some agency (eg your hand) they move toward their home. Earth and water fall and air and fire rise so as to get home. It is obvious that heavy things want to get home more than that light things. Thus a heavy object will move to the earth faster than will light objects. A big stone falls faster than a pea say. Any other motion needs to be explained.

He of course realised that if one throws a lance at an enemy, the lance does not, immediately it has left your hand, fall instantly to the ground. It travels forward as well. Since this is not a natural motion it needs to be explained. Something must cause it move in that way. It was not clear exactly what it was that caused such motion. One idea he had was that it was the air. He realised that the air itself tended to get in the way of such motion, to tend to stop such motion, but he postulated that perhaps it was also the cause of such motion. Somehow the air displaced by the front of the lance went around to the back the back of the lance and pushed it forward. That way there would be a steady and constant force on the lance (or any other object which one threw or was thrown) which kept it moving, in contrast to its natural motion which would be to fall straight down to the ground.

Another of his arguments was against the idea of the void-- ie of a region in which nothing existed. His argument was ingenous, and is in many ways a very modern argument, except that the conclusion was the opposite of what we would make. He argued that if you had a void, empty of anything (including the earth) then if you had something moving in that void, it would never stop. After all, how would it know where to stop? This was absurd, so that everywhere in the sublunary sphere much have some matter or another (the plenum) though which motion takes place

Because of the incredibly strong influence that Aristotle had (he wrote on almost every subject imaginable, and founded a school in the Lycium area of Athens.) Logic, Biology, physics, geography, comedy, Tragedy,....all fell under his gaze in his writing. As a result he (together with Plato and Homer) was one of a very few key authors whose works were consantly copied thoughout the ages, meaning they survived.

He wrote for example on the question as to whether or not the earth rotated-- ie whether the daily motion of the stars across the heavens was due to the motion of the stars, sun, moon and planets aroung the earth, or was due to the motion of the earth spinning on its axis. He came down firmly in the former camp. Part was because of his notion of motion of the earth-- the earth was the earth, not a celestial thing. Its natural motion was rest. How could the earth be moving when the natural state of everything on the earth was rest? But in addition he had other arguments as well. If the earth were rotating, then the natural state of the air was rest and there would be a tremendous wind blowing across the eath. Knowing the size of the earth ( which he estimated) the velocity of the earth would have to be horendous (in modern terms about 1000 miles per hour at the equator) which was clearly nonsense. But his strongest and most convincing argument was that, even if one did not accept the wind argument, if one shot an arrow straight up into the air, the arrow could come back down (in the absense of wind) in the same place where it went up from. But in that time the earth would have moved due to the rotation. Therefor if the earth were rotating the arrow should come down to the west of where it went up. (The rotation of the earth would be from west to east, so things to the west would now be beneath the arrow as it came down).

These arguments convinced most people, including Hipparchus and Ptolemy that the earth could not be rotating. That the things in the heavens must be moving very very fast to go around the earth every day -- well things in heaven are different from things on the earth. The natural motion of the heavens was in circles with constant angular speed.


Even by late antiquity, some of Aristotle's arguments seemed pretty questionable. Philiponus in the 7th century strongly questioned ARistotle's arguments. If one imagined a lance which was sharp at both ends, then there would be nothing for the air to push against. It would simply flow around the sharpened back of the lance. So how could it be that air that kept the lance moving. There must be something in the body itself, some property, some daemon perhaps, that kept it moving. That something must be proportional to the mass of the body, since it is well known that to get something heavy to move is much harder than moving something light.

He also criticised Aristotle;s contention that heavy things fell to earth faster than light things, since a large rock and a small stone both dropped would get the earth at about the same time.

In Europe, Aristotle's work had disappeared. His works were maintained in Byzantium, but due to strong doctrinal and economic intersts, the communication between Byzantium and the "West" were almost non-existant. It was in the 12th to 13th century that Europe rediscovered Aristotle through Islam in Spain. The rulers were very tolerant of religious differences, and allowed Christians and Jews to work and mingle freely (which some extra taxes). The texts of Aristotle had been preserved and studied by Islamic scholars, and translators had gone to Spain to translate some of the text to Latin. Aristotle made a huge impact on thought. Through people like Albertus Magnus and Thomas Aquinus, Arostitelian thought was incorporated into Christian theology, and Aristotle's ideas became the dominant ideas in Christian thought and educated people read him.

Merton School

At the University of Oxford in the mid 1300 a number of scholars in the what we now call the Scholastic tradition, studied Aristotle, and began to think about the whole issue of motion. (Note that the Scholastics have a bad name-- How many angles can dance on the head of pin? This is really unfair. One of the primary concerns was what the relation was between the divine and the mundane. If Christ became man, the divine was both different from and yet similar to the earthly. What are angels, and what is the relation between them and us? What size do they have for example? While their concerns are not of any interest to us now, it is always a mistake to judge past concerns by our own modern standards.)

The Merton school developed some key concepts which were important to later work. They were the first to formulate a concept of velocity or speed of an object as some attribute of the object itself. One of course had the idea of the change of position of an object in time-- now here, now there, and even of the idea of the speed as an concept which embodied that change. But they saw velocity as an attribute that the object possesed in some sense independent of the change in position. The two were of course very closely related but one could think of a body as having a velocity at an instant of time, without the necessity of having to wait for a time to see how the object's position changed.

A second related concept was that this velocity could itself change. Just as a uniform velocity meant that the position of the object changed by the same amount in any periods of equal time, so the velocity could change by the same amount in any equal period of time. Ie, there could be another attribute of the object which was related to velocity in the same way as velocity was to position. And uniform acceleration was the that equal change of velocity in equal times.

Furthermore, the question arose as to what the change in position would be if one had a uniform acceleration, instead of uniform velocity? The "Merton rule" was that if a body started at rest, the distant it travelled in a time was equal to the distance it would have traveled had the body had had a constant velocity equal to the velocity it would have had at half that time. If one had a uniform change in velocity Then at time t, the velocity would be at Ie, if one doubled the time, the velocity would be twice as big. at half the time, the velocity would be at/2. If the object had that same velocity throughout the whole of the time, then the distance travelled would by vt or at(/2) t= att/2

This was a purely mathematical derivation, and had, for them, nothing to do with anything specific in the real world. It would be applied briliantly by Galileo 2 1/2 centuries later.

Buridan and Orseme

Also at around 1350 a strong school with interest in motion occured at the Univ of Paris, under Buridan and his students, one of the most important being Orseme. Buridan was concerned with Aristotle's ideas of motion (just as Philiponus had 700 years earlier). He also realised that air could not be what maintained motion, and he gave the example of an double pointe arrow again. He identified an attribute of the arrow as what he called impetus (we now call it momentum) which was proportional to the mass of the object, and the velocity of the body. It was this internal attribute which was what caused the motion to persist. Exactly how was unclear. Perhaps it was actually some sort of internal demons which kept pushing the object. The air would simply slow down the object.

His student, Orseme, carried on the study of motion. One his key ideas was that the impetus would gradually and on its own, decrease.