The First Table of Elements:
It was believed that earth was some sort of condensation of air, while fire was some sort of emission from air. When earth condenses out of air, fire is created in the process.
This, however was not universally accepted. The most notable detractor was
Democritus who postulated the
existence of indestructible atoms ( from the Greek a-tome:
that which cannot be cut) of an infinite variety of
shapes and sizes. He imagined an infinite universe containing an
infinite number of such atoms, in between
the atoms there is an absolute void. Aristotle's universe is
Aristotle concluded the atomists were wrong, stating that matter is in fact continuous and infinitely divisible.
Aristotle's cosmological work On The Heavens is the most influential treatise of its kind in the history of humanity. It was accepted for more that 18 centuries from its inception (around 350 B.C.) until the works of Copernicus in the early 1500s. In this work Aristotle discussed the general nature of the cosmos and certain properties of individual bodies.
The idea that all bodies, by their very nature, have a natural way of moving is central to Aristotelian cosmology. Movement is not, he states, the result of the influence of one body on another so, no Physics !
Movement is therefore endowed to bodies. Is this science?
Some bodies naturally move in straight lines, others naturally are at rest (but what does that mean?). But there is yet another natural movement: the circular motion.
By logic: since to each motion there must correspond a substance, there ought to be some bodies that naturally move in circles. Aristotle then proclaims that such things are the heavenly bodies as they are made of a more exalted and perfect substance than all earthly objects.
Note the desciptors here: Exalted and Perfect
Since the stars and planets are made of this exalted substance and then move in circles, it is also natural, according to Aristotle, for these objects to be spheres also. The cosmos is then made of a central earth (which he accepted as spherical) surrounded by the moon, sun and stars all moving in circles around it. This conglomerate he called ``the world''.
(The Finite Crystalline Sphere Universe)
The initial motion of these spheres was caused by the action of a prime mover which acts on the outermost sphere of the fixed stars; somehow, however, this action is communicated to the other spheres and they move as well, but at different rates, since that is what the observations demand.
Aristotle also asserts that the world did not come into being at one point, but that it has existed, unchanged, for all eternity (it had to be that way since it was perfect .
Still, since he believed that the sphere was the most perfect of the geometrical shapes, the universe did have a center (the Earth) and its material part had an edge, which was ``gradual'' starting in the lunar and ending in the fixed star sphere.
Beyond the sphere of the stars the universe continued into the spiritual realm where material things cannot be.
This is in direct conflict with the Biblical description of creation, and an enormous amount of effort was spent by the medieval philosophers in trying to reconcile these views (and, for which, of course, no reconciliation is possible).
Discussion question: What is the "physics" problem with the Universe having an infinite age?
Cliff Notes Summary: Starting from the hypothesis that heavenly bodies have a natural circular motion, Aristotle postulated that all such bodies move in spheres around the Earth, Aristotle painstakingly modified this idea, matching it to the observations, until all data could be accurately explained. He then used this theory to make predictions (such as where will Mars be next "year") which were confirmed by subsequent observations. This gave the model real power. There is also no credible observations that could disprove this model, at the time. There are only some observations which are slightly inconsistent.
The Cause of Motion:
One of the fundamental propositions of Aristotelian philosophy is that there is no effect without a cause.
Applied to moving bodies, this proposition dictates that there is no motion without a force. Speed, then is proportional to force and inversely proportional to resistance (though none of these quantites are unambiguously defined).
Is this a resonable proposition?
Qualitatively this implies that a body will traverse a thinner medium in a shorter time than a thicker medium (of the same length): things will go faster through air than through water.
Class Discussion Question: What is the logical consequence of motion in a vacuum, under this hypothesis?
For falling bodies, the force is the weight pulling down a body and the resistance is that of the medium (air, water, etc.). Aristotle noted that a falling object gains speed, which he then attributed to a gain in weight.
If weight determines the speed of fall, then when two different weights are dropped from a high place the heavier will fall faster and the lighter slower, in proportion to the two weights. A ten pound weight would reach the Earth by the time a one-pound weight had fallen one-tenth as far.
Let's test this hypothesis!
It seems very curious that we had to wait until Galileo showed this to be preposterous.
According to Aristotle, there is a privileged being: The Prime Mover. He is the first agent, responsible for moving objects, which, in turn, move other objects.
The Prime Mover, he argued, must be at Absolute Rest. By "absolute" rest , we mean that all observers will universally agree on that state of rest.
Class Discussion Point
Read this passage from Aristotle and state what you think it means?
|That a final cause may exist among unchangeable entities is shown by the distinction of its meanings. For the final cause is (a) some being for whose good an action is done, and (b) something at which the action aims; and of these the latter exists among unchangeable entities though the former does not. The final cause, then, produces motion as being loved, but all other things move by being moved. Now if something is moved it is capable of being otherwise than as it is. Therefore if its actuality is the primary form of spatial motion, then in so far as it is subject to change, in this respect it is capable of being otherwise,--in place, even if not in substance. But since there is something which moves while itself unmoved, existing actually, this can in no way be otherwise than as it is. For motion in space is the first of the kinds of change, and motion in a circle the first kind of spatial motion; and this the first mover produces. The first mover, then, exists of necessity; and in so far as it exists by necessity, its mode of being is good, and it is in this sense a first principle. For the necessary has all these senses--that which is necessary perforce because it is contrary to the natural impulse, that without which the good is impossible, and that which cannot be otherwise but can exist only in a single way. (Metaphysics, Book XII, Chapter 7, 1072)|
Projectile Motion: In Aristotle's view, objects moved parallel to the Earth's surface until it was time (i.e. it was their destiny) to fall back to the earth
Interesting historical experiment proving this completely wrong but the importance of that seems to have been lost:
Archimedes of Syracuse (287 - 212 BCE) took a more practical approach. At the request of Hieron, the tyrant of Syracuse and a family friend, Archimedes interrupted his mathematical researches long enough to apply his understanding of levers and pulleys to the construction of remarkable catapults and cranes to defend the city against Roman attack. Archimedes' catapults lobbed quarter-ton stones at the attacking Roman ships, quickly destroying the clumsy Roman siege engine.
Had physics worked the way Aristotle said, hitting targets with catapults would be damn difficult!