A Cut and Paste History of the Sciences of Motion Selections are about or by Aristotle, Strato, Aristarchus, Philoponus, Buridan, Hobbes, Descartes, Newton.
Harry ----------------- Aristotle's prime mover. [My comment: It is different from the prime mover of the mechanical philosopher's which is really a prime pusher.] >From History of Philosophy, Vol.I, F.Copleston, Image Books Doubleday, New York, 1974 If God caused motion by efficient physical causation, then He Himself would be changed: there would be a reaction of the moved on the mover. He must act, therefore, as Final Cause, by being the object of desire. The moving Principle must be of such a kind that it is pure act, without potential. (http://www.theologywebsite.com/history/aristotle.shtml) The prime mover is not a causal agent in an active sense. it moves other things by being an object of their desire: they desire its supreme perfection and thus are moved ------------------- From, http://galileoandeinstein.physics.virginia.edu/lectures/lecturelist.html Michael Fowler UVa Physics Strato As we mentioned before, Aristotle's analysis of motion was criticized by Strato (who died around 268 B.C., he is sometimes called Straton), known as "the Physicist" who was the third director of the Lyceum after Aristotle (the founder) and Theophrastus, who was mainly a botanist. Strato's career was curiously parallel to Aristotle's. Recall Aristotle spent twenty years at Plato's academy before going to Macedonia to be tutor to Alexander, after which Aristotle came back to Athens to found his own "university", the Lyceum. A few years later, Alexander conquered most of the known world, dividing it into regions with his old friends in charge. In particular, he had his boyhood friend Ptolemy in charge of Egypt, where Alexander founded the new city of Alexandria. Now Strato, after a period of study at the Lyceum, was hired by Ptolemy to tutor his son Ptolemy II Philadelphus (as he became known) in Alexandria. Subsequently Strato returned to Athens where he was in charge of the Lyceum for almost twenty years, until his death. Strato, like Aristotle, believed in close observation of natural phenomena, but in our particular field of interest here, the study of motion, he observed much more carefully than Aristotle, and realized that falling bodies usually accelerate. He made two important points: rainwater pouring off a corner of a roof is clearly moving faster when it hits the ground than it was when it left the roof, because a continuous stream can be seen to break into drops which then become spread further apart as they fall towards the ground. His second point was that if you drop something to the ground, it lands with a bigger thud if you drop it from a greater height: compare, say, a three foot drop with a one inch drop. One is forced to conclude that falling objects do not usually reach some final speed in a very short time and then fall steadily, which was Aristotle's picture. Had this line of investigation been pursued further at the Lyceum, we might have saved a thousand years or more, but after Strato the Lyceum concentrated its efforts on literary criticism. Aristarchus Strato did, however, have one very famous pupil, Aristarchus of Samos (310 - 230 B.C.). Aristarchus claimed that the earth rotated on its axis every twenty-four hours and also went round the sun once a year, and that the other planets all move in orbits around the sun. In other words, he anticipated Copernicus in all essentials. In fact, Copernicus at first acknowledged Aristarchus, but later didn't mention him (see Penguin Dictionary of Ancient History). Aristarchus' claims were not generally accepted, and in fact some thought he should be indicted on a charge of impiety for suggesting that the earth, thought to be the fixed center of the universe, was in motion (Bertrand Russell, quoting Plutarch about Cleanthes). The other astronomers didn't believe Aristarchus' theory for different reasons. It was known that the distance to the sun was in excess of one million miles (Aristarchus himself estimated one and a half million miles, which is far too low) and they thought that if the earth is going around in a circle that big, the pattern of stars in the sky would vary noticeably throughout the year, because the closer ones would appear to move to some extent against the background of the ones further away. Aristarchus responded that they are all so far away that a million miles or two difference in the point of observation is negligible. This implied, though, the universe was really huge -- at least billions of miles across -- which few were ready to believe. ------------- John Philoponus, a Christian philosopher, scientist, and theologian who lived approximately from 490 to 570 http://plato.stanford.edu/entries/philoponus/#2.2 Theory of Impetus The Physics commentary contains an array of examples of innovative and damagingly critical commentary. One of the most celebrated achievements is the theory of impetus, which is commonly regarded as a decisive step away from an Aristotelian dynamics towards a modern theory based on the notion of inertia. Concepts akin to those deployed in Philoponus' impetus theory appear in earlier writers such as Hipparchus (2nd c. BCE) and Synesius (4th c. CE), but Philoponus nowhere intimates that he was influenced by any one of them. As far as one can tell from the text In Phys. 639-42, he takes his point of departure from an unsatisfactory Aristotelian answer to a problem that was to puzzle scientists for centuries: Why does an arrow continue to fly after it has left the bow-string, or a stone after it has ceased to be in contact with the hand that throws it? Since Aristotle supposed that a) whenever there is motion there must be something which imparts the motion, and b) mover and moved must be in contact, he was led to conclude that the air displaced in front of the projectile somehow rushes round it and pushes from behind, thus propelling the projectile along. This theory was still in vogue among Aristotelians of the sixteenth century, despite the fact that a thousand years earlier Philoponus had had no truck with it. He proposed instead, much more plausibly but still erroneously, that a projectile moves on account of a kinetic force which is impressed on it by the mover and which exhausts itself in the course of the movement. Philoponus compares this impetus or incorporeal motive enérgeia¹, as he calls it, to the activity earlier attributed to light. Once projectile motion was understood in terms of an impetus in this way, it became possible for Philoponus to reassess the rôle of the medium: far from being responsible for the continuation of a projectile's motion it is in fact an impediment to it (In Phys. 681). On this basis Philoponus concludes, against Aristotle, that there is in fact nothing to prevent one from imagining motion taking place through a void. As regards the natural motion of bodies falling through a medium, Aristotle's verdict that the speed is proportional to the weight of the moving bodies and indirectly proportional to the density of the medium is disproved by Philoponus through appeal to the same kind of experiment that Galileo was to carry out centuries later (In Phys. 682-84) ---------------------------- http://www.upscale.utoronto.ca/GeneralInterest/Key/mediaeva.htm#Buridan Jean Buridan (1295?-1366?) Buridan was one of the most influential teachers of his time, whose ideas were still being taught at universities as late as the 17th century. He was a scholar of wide interests, publishing textbooks on almost every subject taught at the University of Paris where he was rector. Much of his work in the area of natural philosophy consisted of interpretations and commentaries on the works of Aristotle, which he attempted to assimilate to contemporary ideas. His work was prohibited from 1474 to 1481. It is probable that he died of the plague. He is well-known in philosophical circles for his discussion of Buridan's ass which, placed equidistant from two bales of hay, died from an inability to decide which one to eat first. Buridan was a strong proponent of the Impetus Theory of motion, which had been recently revived by William of Occam. In much the same way as a body that has been heated possesses a quantity of heat, the impetus theory suggested that a moving body possessed a quantity of motion (the impetus), imparted to it by the original force, and proportional to the mass of the body and its initial imparted speed. Thus a body would continues to move though a fluid until its impetus was exhausted by the resistance of the fluid. The Impetus Theory allowed motion in a vacuum (which Aristotle's did not). It also predicted that a body, moving in a circle under the influence of a centripetal force (e.g. think of whirling a mass tied to a string around your head) would continue to move in a circle for a short time after the force was removed. According to this theory, projectile motion would consist of three parts: (i) the body would move horizontally, the impetus suppressing any effect of gravity: (ii) a brief period of compromise between the impulse and gravity: (iii) the projectile falls vertically in "natural" motion. God was supposed to have given the planets a starting (circular) impetus, which, in the absence of air resistance, kept them going for ever. Buridan's is a good example of the many different versions of the Impetus theory that held sway during the Middle Ages. --------------------- IMPETUS THEORY Jean Buridan http://www.physics.vanderbilt.edu/astrocourses/ast203/impetus_theory.html Quote from Buridan's "Quaestiones on Aristotle's Physics": "When a mover sets a body in motion he implants into it a certain impetus, that is, a certain force enabling a body to move in the direction in which the mover starts it, be it upwards, downwards, sidewards, or in a circle. The implanted impetus increases in the same ratio as the velocity. It is because of this impetus that a stone moves on after the thrower has ceased moving it. But because of the resistance of the air (and also because of the gravity of the stone) which strives to move it in the opposite direction to the motion caused by the impetus, the latter will weaken all the time. Therefore the motion of the stone will be gradually slower, and finally the impetus is so diminished or destoyed that the gravity of the stone prevails and moves the stone towards its natural place. In my opinion one can accept this explanation because the other explanations prove to be false whereas all phenomenaa agree with this one." Note that the implanted impetus is caused by a mover who imparts an initial velocity to a projectile; the impetus is proportional to the velocity: in fact, Buridan gave it a mathematical formulation: impetus = weight x velocity Notes on figure [Not Available] illustrating ballistic trajectory of impetus theory vs. Aristotelian theory: Three stages of projectile motion yield the ballistic curve in the illustration [N.A.]: 1. Initial stage. Impetus is dominant. Gravity is insignificant. Motion is in a straight line in direction of impetus. 2. Intermediate stage. Air resistance slows projectile. Gravity recovers. Path begins to deviate downwards. Path deviates downwards from straight line. This part of the path was conceived as part of a great circle. 3. Last stage. Impetus is completely spent. Gravity alone draws projectile downwards. Compare this with an Aristotelian trajectory: 1. Impetus comes from surrounding air, which receives it from the pusher (a catapult, for example). This results in a straight line trajectory, with decreasing velocity. 2. At a certain point, the force is exhausted so the projetile falls downwards in a straight line. Is impetus theory a forerunner of Momentum, ala Galileo and Newton? Yes and No. Yes: Like momentum, impetus, once imparted to an object, will endure forever unless corrupted by an outside force. With impetus theory, angels are not needed to push celestial spheres. With an initial impetus, spheres would keep moving since there is no air resistance in the celestial realm. No: Not like inertia: in modern inertia, rest and motion are equivalent. But impetus has no meaning for a non-moving object. No: In modern theory, we speak of both linear and angular (circular motion) momentum. But angular motion requires a force to be maintained (in modern theory). Buridan used impetus theory to explain LINEAR as well as CIRCULAR motion, i.e., these were essentially the same; impetus was the force that tended to uphold the INITIAL motion, whether straight or circular; this idea survived for 300 years until Galileo. What does Impetus Theory mean for astronomy? LIBERATION. Freed from domination by Aristotle's laws of motion, astronomers can pursue new ideas. We don't need angels to push celestial spheres around; as written by Buridan about celestial intelligences: "one could imagine that it is unneccesary to posit intelligences as the movers of celestial bodies since the Holy Scriptures do not inform us that intelligences must be posited. For it could be said that when God created the celestial spheres, He began to move each of them as He wished, and they are still moved by the IMPETUS which He gave to them because, there being no resistance, the impetus is neither corrupted nor diminished." This theory of heavenly motion is a radical break with the traditional view. Traditionally, back to Aristotle, celestial and terrestrial phenomena were made of different stuff and so obeyed related but separate laws of physics; the impetus theory enabled philosophers to include celestial motion into the same theory used to describe terrestrial motion. Yet, though impetus theory appears sensible in many ways, it is in contradiction so many things that are observed. Common sense says Aristotle might still be right. So, even Buridan retains the traditional view of solid celestial spheres (not planets) being the objects in motion. And Oresme ultimately believed that angles moved the celestial spheres. ------------- Hobbes http://www.bu.edu/wcp/Papers/Mode/ModePiet.htm Juhani Pietarinen University of Turku, Finland I. Conatus and Motion Philosophers in the 17th century made hard efforts to explain the beginning and continuation of the motion of bodies. The notion of conatus ('striving' or 'endeavoring') was commonly used in the explanations. It refers to the power with which the motion of a body begins and is kept on. What is this power? Descartes explained it to be an active power or tendency of bodies to move, expressing the power of God. He distinguished between motion and the tendency to move, but Hobbes was anxious to argue that conatus actually is motion. In The Elements of Law he says it to be the "internal beginning of animal motion" (EL I.7.2), and in his later writings the notion of 'endeavor' refers to the beginning or first part of any kind of motion. Because motion is for Hobbes "a continual relinquishing of one place, and acquiring of another" (De Corp II.8.10), the beginning of a motion of a body must be an infinitely small change in the place of the body. Accordingly, Hobbes defines endeavor "to be motion made in less space and time than can be given; ... that is, motion made through the length of a point, and in an instant or point of time" (De Corp III.15.2). For Hobbes, the conatus is not an inherent power of a body but is determined by the motions of other bodies. However, he regards it as an active power, because "the beginning of the motion of a body must be considered as action or cause" (De Corp II.9.6). Thus endeavor is the power by which a body affects the motion of other bodies and resists their power, and, in a sense, also 'causes' the motion of the body itself, for Hobbes takes the principle of the persistence of motion to be true: "whatsoever is moved, will always be moved in the same way, and with the same swiftness, if it be not hindered by some other moved and contiguous body" (De Corp III.15.1). Thus Hobbes, like Descartes and Spinoza, takes conatus to be the active power by which a body persists in its state of motion. In brief, Hobbes accepts the following fundamental principle: (CP) The conatus-principle: A body endeavors to preserve its state and resist the causal power of other bodies. This is a true natural law for Hobbes. I want to show the importance of (CP) for Hobbes's theory of human action and political philosophy. ------------------------------- Descartes' The World http://www.princeton.edu/~hos/mike/texts/descartes/world/worldfr.htm ...Now it is the case that those two rules manifestly follow from this alone: that God is immutable and that, acting always in the same way, He always produces the same effect. For, supposing that He placed a certain quantity of motions in all matter in general at the first instant He created it, one must either avow that He always conserves as many of them there or not believe that He always acts in the same way. Supposing in addition that, from that first instant, the diverse parts of matter, in which these motions are found unequally dispersed began to retain them or to transfer them from one to another according as they had the force to do, one must of necessity think that He causes them always to continue the same thing. And that is what those two rules contain. I will add as a third rule that, when a body is moving, even if its motion most often takes place along a curved line and (as has been said above) can never take place along any line that is not in some way circular, nevertheless each of its individual parts tends always to continue its motion along a straight line. And thus their action, i.e. the inclination they have to move, is different from their motion. For example, if a wheel is made to turn on its axle, even though its parts go around (because, being linked to one another, they cannot do otherwise), nevertheless their inclination is to go straight ahead, as appears clearly if perchance one of them is detached from the others. For, as soon as it is free, its motion ceases to be circular and continues in a straight line. By the same token, when one whirls a stone in a sling, not only does it go straight out as soon as it leaves the sling, but in addition, throughout the time it is in the sling, it presses against the middle of the sling and causes the cord to stretch. It clearly shows thereby that it always has an inclination to go in a straight line and that it goes around only under constraint. This rule rests on the same foundation as the two others and depends only on God's conserving everything by a continuous action and, consequently, on His conserving it not as it may have been some time earlier but precisely as it is at the same instant that He conserves it. Now it is the case that, of all motions, only the straight is entirely simple; its whole nature is understood in an instant. For, to conceive of it, it suffices to think that a body is in the act of moving in a certain direction, and that is the case in each instant that might be determined during the time that it is moving. By contrast, to conceive of circular motion, or of any other possible motion, one must consider at least two of its instants, or rather two of its parts, and the relation between them.[37] But, so that the philosophers (or rather the sophists) do not find occasion here to exercise their superfluous subtleties, note t at I do not thereby say that rectilinear motion can take place in an instant; but only that all that is required to produce it is found in bodies in each instant that might be determined while they are moving, and not all that is required to produce circular motion... ----------- Newton's definition of inertia >From his Principia: Definition III The vis insita, or innate force of matter, is a power of resisting, by which every body, as much as in it lies, continues in its present state, whether it be of rest, or of moving uniformly forwards in a right line. This force is always proportional to the body whose force it is and differs nothing from the inactivity of the mass, but in our manner of conceiving it. A body, from the inert nature of matter, is not without difficulty put out of its state of rest or motion. Upon which account, this vis insita may, by a most significant name, be called INERTIA (vis inertiae) or force of inactivity. But a body only exerts this force when another force, impressed upon it, endeavours to change its condition; and the exercise of this force may be considered as both resistance and impulse; it is resistance so far as the body for maintaining its present state, opposes the force impressed; it is impulse so far as the body, by not easily giving way to the impressed force of another endeavours to change the state of that other. Resistance is usually ascribed to bodies at rest, and impulse to those in motion; but motion and rest, as commonly conceived, are only relatively distinguished; nor are those bodies always truly at rest, which commonly are taken to be so. ------------
