2.3. The Mechanics of Movement

Aristotle had had a problem with explaining the mechanics of moving objects. If a person throws an object, he/she causes it to move; but it keeps moving after the person has stopped moving it (Cattell 2006: 7).

Aristotle

How can this be (moving after the person has stopped moving it)? Aristotle’s answer was that the medium in which the object moves sustains the movement. But he did not succeed in explaining clearly what he meant by that.

Let us see his attempt below:

... although the first cause of movement imparts the ability to cause movement to the air or the water (or whatever else it may be that is, by its nature, capable of causing movement and of being moved), nevertheless the air or water or whatever does not stop causing movement and being moved at the same time as the first mover stops; it may stop being moved as soon as the cause of movement stops imparting movement, but it retains its ability to cause movement. That is why it imparts movement to something else which is consecutive to it, and the same goes for this in turn. (Aristotle 1996: 229)

Galileo

Galileo made a considerable leap forward with this problem, when he claimed that if a projectile were impelled through a medium that presented no resistance, it would continue forever. Descartes, following Galileo, made much the same point.

He says:

... each thing, in so far as it is simple and undivided, always remains in the same state as far as it can, and never changes except as a result of external causes. (Descartes 1985, Volume I. Principle 37: 240-241)

In the discussion, he points out that square things remain square, unless something changes them. Bodies at rest remain at rest, unless something causes them to move.

and if it move, there is equally no reason for thinking it will ever lose this motion of its own accord and without being checked by something else. Hence, we must conclude that what is in motion always, so far as it can, continues to move. (Ibid., pp. 241)

Newton and Hooke

According to Herival (1965: 153), following Descartes, Newton enunciated the principle that “everything continues in ‘the state in which it is’ unless it is interrupted by some external force (cause). Hence, ‘[a] body once moved will always keepe y^e same celerity, quantity & determination of its motion.” But although Newton thus drew on Descartes’ principle of inertia, he did not mention Descartes’ law of impact, which was the culmination of Descartes’ discussion of motion. Instead, Newton set forth his own theory of impact.

For Descartes, force had been internal to the moving object, but for Newton force was external to the object, which was the passive recipient of the force.

Newton had the important insight that there was a mathematically exact relationship between the force that one body exerted on another and the consequent motion (or reduction of motion) that it effected, as pointed out by Herival (1965):

A body is said to have more or less motion as it is moved with more or less force, that is as there is more or less force required to generate or destroy its whole motion.” (pp. 157)

It was when Newton started dealing seriously with circular motion that he made real progress in his thinking about force. Since he believed that a body moving in a straight line would continue to do so forever unless it was affected by some outside force, it seemed that anybody that moved in circle must be under the influence of some force that diverted it from a straight path. His first idea was that a body in circular motion must be under the influence of two opposite forces, one towards the centre and an equal one away from it, so that the two are in equilibrium.

It was Newton’s rival Robert Hooke who made the breakthrough in correctly describing orbital motion, when he announced that his forthcoming description of a system of the world depended on the following supposition (together with two others):

That all Celestial Bodies whatsoever, have an attraction or gravitating power towards their own Centers, whereby they attract not only their own parts, and keep them from flying from them, as we may observe the earth to do, but that they do also attract all other Coelestial Bodies that are within the sphere of their activity. (Gunther 1931: 27-8)

Hooke had expressed the central insight about orbital motion, that it involved centripetal force. There has been much argument as to whether this passage also expresses the idea of universal gravitation.

The idea of universal gravitation, of course, is that any two objects attract each other. Newton is usually credited with this discovery, in spite of Hooke’s complaints. He is also given credit for two other related discoveries: that the greater the mass of the objects is, the greater the attraction is between them; and that the further away from each other they are, the weaker the attraction is between them.

It is noted that the idea of ‘matter’ had changed radically in the process. A few years earlier, those scholars, including Descartes, who had been followers of mechanism, had insisted that one object could exert an influence on another only if they were in contact. Now Newton was talking of attraction between bodies that were separated by large distances.

In his Perspectives on Power, Chomsky (1997: 6-7) comments that the mechanical philosophy pursued by Descartes:

...collapsed within a generation. Newton demonstrated that the world is not a machine. Rather, it has occult forces after all. Contact mechanics simply does not work for terrestrial and planetary motion. Some mystical concept of ‘action at a distance’ is required. Newton was harshly criticized by leading scientists of the day for retreating to mysticism and undermining the achievements of the mechanical philosophy. He seems to have agreed, regarding the idea of action at a distance as an ‘absurdity’, though one must come to terms somehow with the refutation of the mechanical philosophy.

So, despite the progress that genius had made, the problem of the nature of matter had not really been solved; nor has a final solution been arrived at in the twenty-first century.