This entry presents an overview on how mechanics in Greece was linked to geometry. In ancient Greece, mechanics was about lifting heavy bodies, and mathematics almost coincided with geometry. Mathematics interconnected with mechanics at least from the 5th century BCE and became dominant in the Hellenistic period. The contributions by thinkers such as Aristotle, Euclid, and Archytas on fundamental problems such as that of the lever are sketched. This entry can be the starting point for a deeper investigation on the connections of the two disciplines through the ages until our present day.
[.] as the mathematician investigates abstractions (for before beginning his investigation he strips off all the sensible qualities, e.g., weight and lightness, hardness and its contrary, and also heat and cold and the other sensible contrarieties, and leaves only the quantitative and continuous, sometimes in one, sometimes in two, sometimes in three dimensions, and the attributes of these qua [i.e., inasmuch] quantitative and continuous, and does not consider them in any other respect, and examines the relative positions of some and the attributes of these, and the commensurabilities and incommensurabilities of others, and the ratios of others; but yet we say there is one and the same science of all these things—geometry), the same is true with regard to being.[3] (11, 3. 27)
Similar evidence is supplied by the more physical of the branches of mathematics, such as optics, harmonics, and astronomy. These are to some extent the converse of geometry. While geometry investigates physical lines but not qua physical, optics investigates mathematical lines, but qua physical, not qua mathematical.[4] (II, 194a, 7 ff.)
The same account may be given of harmonics and optics; for neither considers its objects qua sight or qua voice, but qua lines and numbers; but the latter are attributes proper to the former. And mechanics too proceeds in the same way.[3] (M, 3, 1078a)
He was the first to bring mechanics to a system by applying mathematical principles; he also first employed mechanical motion in a geometrical construction, namely, when he tried, by means of a section of a half-cylinder, to find two mean proportionals in order to duplicate the cube. In geometry, too, he was the first to discover the cube, as Plato says in the Republic.[5] (volume 2, book 8, 83, pp. 395–396)
One marvels at things that happen according to nature, to the extent the cause is unknown, and at things happening contrary to nature, done through art for the advantage of humanity. Nature, so far as our benefit is concerned, often works just the opposite to it. For nature always has the same bent, simple, while use gets complex. So whenever it is necessary to do something counter to nature, it presents perplexity on account of the difficulty, and art [τεχνη, techne] is required. We call that part of art solving such perplexity a mechane.[11] (p. 1)
Now if of two objects moving under the influence of the same force one suffers more interference, and the other less; it is reasonable to suppose that the one suffering the greater interference should move more slowly than that suffering less.[6] (p. 341)
and KH, parallel to the tangent to the circumference, as ‘according to nature’; he labels the motions XZ, BY along the radius of the same circumference, as ‘against nature’. This denomination makes it natural in referring to Figure 1a, where the ‘natural’ motion is vertical and directed downward, just like that of a falling weight, to which Aristotle referred constantly in his philosophy of nature.
Figure 1. Vertical (a) and horizontal (b) motion according to nature
Geometry proves that, in correspondence with the same natural motion Z, themotion against nature decreases as the radius of the circle increases, since YB < ZX. Basically, what Aristotle said is that a body under a given force and constrained to move along a circumference moves faster the greater the radius of the circumference, because the deviation (the interference to motion) is smaller.
The other term that needs to be justified is force (ισχν'ος). The term used by Aristotle seems to refer to muscular force but on some occasions, more or less explicitly, identifies weight, which, like muscular force, exerts a dragging action. One can then paraphrase Aristotle saying that “a body of given weight constrained to move along a circumference moves faster the greater the radius of the circumference”.
[.] the ratio of the weight moved to the weight moving it is the inverse ratio of the distances from the centre.[6] (p. 353)
The reason has been given before that the point further from the centre describes the greater circle, so that by the use of the same force, when the motive force is farther from the lever [sic! correct: fulcrum], it will cause a greater movement.[6] (p. 353)
However, also accepting this view, one can correctly formulate only this rule: “a weight p located at a distance d > D from the fulcrum can raise a weight P > p located at a distance D”, which is an indeterminate law, as remarked by Bernardino Baldi (1553-1617 CE):
Thus when Aristotle discloses the reason for which the lever moves a weight more easily, he says that this happens because of the greater length on the side of the power that moves; and this [accords] quite well with his first principle, in which he assumes that things at the greater distance from the centre are moved more easily and with greater force, from which he finds the principal cause in the velocity with which the greater circle overpowers the lesser. So the cause is correct, but it is indeterminate; for I still do not know, given a weight and a lever and a force, how I must divide the lever at the fulcrum so that the given force may balance the given weight. Therefore Archimedes, assuming the principle of Aristotle, went on beyond him; nor was he content that the force be on the longer side of the lever, but he determined how much [longer] it must be, that is, with what proportion it must answer the shorter side so that the given force should balance the given weight.
For the indeterminate law to become determinate, one needs to replace the relations of greater or lesser with an equality and to allow the variation of the value of the moving weight, so as to give: “a heavy body P brings about an action on another body Q, located on the opposite side of a lever, which is directly proportional to its distance d from thefulcrum and to its weight p”; in modern terms, it is the product dp that actually counts.
It is worth remarking that in the explanation of the law of the lever the circumstance that “the mover will manage more the farther from the fulcrum” is stressed more than the balance between the tendency to go down of the two weights on the lever. This is the same occurring in the Middle Age treatise Ratione ponderis attributed to Jordanus Nemorarius (13th century CE) [13] (p. 84).
Assuming the lever of Figure 2, one can accept that there is an equilibrium when two weights G are located at the same distance from the fulcrum A (that is, AB = AM) for symmetry reasons. However, the weight G at M can be replaced by a weight D at a distance AE such that AE × D = AM × G = AB × G, which is the law of the lever in its standard formulation.
Figure 2. The law of the lever.
The treatise is very terse, completely different from the wordy Mechanica Problemata. As usual in Greek writings on geometry, there are definitions (1), axioms (2), and propositions (4), not present in the Aristotelian treatise. Apart from the logical structure, the main difference with the Mechanica Problemata is that here there are no references to principles of a pre-established philosophy of nature, as it occurs also for a modern treatise on physical mathematics, after two thousand years. Moreover, the axioms in The Book of the Balance, although not necessarily shared by everyone at a first reading, are verifiable with simple experiments, even mental ones.
The definition and the axioms are referred to below:
Definition 1. Weight is the measure of the heaviness and lightness of one thing compared to another by means of a balance.
Axiom 1. When there is a straight beam of uniform thickness, and there are suspended on its extremities two equal weights, and the beam is suspended on an axis at the middle point between the two weights, then the beam will beparallel to the plane of the horizon.
Axiom 2 . When two weights—either equal or unequal—are placed on theextremities of a beam, and the beam is suspended by an axis on some position of it such that the two weights keep the beam on the plane of the horizon, then if one of the two weights is left in its position on the extremity of the beam and from the other extremity of the beam a straight line is drawn at a right angle to the beam in any direction at all, and the other weight is suspended on any point at all of this line, then the beam will be parallel to the plane of the horizon as before [. . . ]
[19] (p. 220. Translation into English in [18])
At least two other axioms, very intuitive indeed, are implicit:
Axiom 3’. Suspending any weight at the fulcrum, if the balance is horizontal it will remain horizontal.
Axiom 4’. Weight is an additive measure. That is, given two heavy bodies with equal weight, the heavy body composed by the two bodies has a double weight.
Figure 3. Displacement of weights orthogonal to the beam in a vertical (a) and a horizontal (b) plane.
Proposition 1. This being assumed, we pose a straight line AB (see Figure 4) as a beam of a balance whose axis is at point C, and we draw CE at a right angle to line AB, and we extend it in a straight line to point D, and we make line CD equal to CE, and we complete the square CH by drawing parallels. Then we place equal weights at points A, H and E. And so I say that these three weights keep lines AB, ED parallel to the horizon.The proof is this: tne weight has been placed on one of the extremities of line AB at point A. From the other extremity we have drawn a line at right angles, the line BH, and we have placed on it a weight equal to the weight which is at point A. And so the two weights maintain the line AB parallel to the horizon [by Axiom II]. For the same reason it is necessary that the two weights which are at points E, H keep line ED parallel to the horizon. Thus weights A, E, H will keep lines AB, ED parallel to the horizon.It is clear that if the weight which is at point H is removed to point B from which the line BH was drawn at right angles, then with weight A it maintains line AB parallel to the horizon, just as it was necessary when the weight was at point H. The line ED will accordingly not be in equilibrium, since the weight E will make it incline on its side. But if weight E is moved to point C, or if weight E is left on its place and a weight equal to it is placed at point D, then the weight E balances the line ED and it will be parallel to the horizon. We conclude from this that the weight which is at point H was one weight which stood in place of two weights at points B, D, each of which was equal to it.
Figure 4 illustrates the configuration of the lines ACB, CDE, BH, and DH drawn on a horizontal plane, so that the direction of gravity is perpendicular to the same plane. The proof of Proposition 1 is simple, even if it is not clear what its role is in the proof of the law of the lever. This approach is different from that in Aristotle’s text and is typical of Greek geometry, in which without an initial comment explaining the strategy, a series of propositions are shown, the purpose of which is revealed only at the end.
Figure 4. Equilibrium of a beam in the space. Redrawn from [19], p. 221.
Proposition 2 follows immediately from Proposition 1 and states that the equal arm balance of Figure 5a, with the fulcrum in C, is in equilibrium under three equal weights, one placed at the free end H, one at a distance CZ from the fulcrum, and the last at a distance TW from the free end H, so that CZ = WT. Then, since CZ and WT are arbitrary, the proposition allows us to affirm more generally that in a balance in equilibrium under three equal weights such as that in Figure 5a, the two weights hanging from the same arm can be shifted in opposite directions by the same amount (notice that if there is equilibrium for CZ = WT, there cannot be equilibrium for CZ 6= WT).
Axiom 5’. If any number of weights keep a balance in the horizontal position and two weights hanging at one of the two arms are moved in the opposite direction of the same amount, the balance still remains in the horizontal position.
At this point, the law of the lever can be easily proved in Proposition 4, with reference to Figure 6a. The balance HCE is in equilibrium by Axiom 1 if two equal weights p are hung at H and E and two weights are suspended below the fulcrum C.
Figure 6. The law of the lever for a particular case: (a) first step of the proof; (b) second step of the proof; (b) is redrawn from [18], p. 27.
Due to Proposition 2, we can move the weight p in C to position A and the weight p in E to position Z without altering equilibrium (we could already start from the position with the weights in A and Z, in equilibrium according to Proposition 2). Then, one can repeat the operation moving another weight p from C to A and the weight p from Z to A; this can be performed in accordance with Axiom 5’ but not with Proposition 2, because on the arm CE, there are more than two weights. Thus, we obtain the configuration of Figure 6b with three equal weights in A, which for Axiom 4’ are equivalent to a weight having a triple value of the weight in B. The law is thus proved for a weight ratio of 3 : 1, and Euclid leaves to the reader the generalization of the law of the lever to the more general case of the ratio of weights expressed by any pair of integers.
A separate discussion is needed for Proposition 3, which is enunciated and proven by means of Figure 7, illustrating a balance BCA of equal arms with fulcrum in C, in equilibrium under three equal weights applied in B, Z, and D. The 10 segments into which BA is divided are all equal.
Figure 7. The force of weight. Redrawn from [19], p. 223.
Proposition 3 states that one can shift the weight at B onto T and the weight at D onto E, still keeping the balance horizontal. It is in fact a shorter balance than the previous one, for which Proposition 2 still holds. The same holds good if one displaces the weight at T onto H and the weight at D into E. The proposition expresses the fact, which is proved for the particular case of Figure 7, that in a balance with three weights it is possible to move each of two equal weights hanging from different arms in opposite directions along the arms, without altering the horizontality of the balance.
It is then clear that the diminution of force of weight when the weight is moved from B to T is equal to the diminution that occurs when a weight is moved from T to H. The same reasoning applies to all the quantities of equal lengths taken from CB.
This statement suggests to generalize Axiom 5’, by removing the clause “hung at one of the two arms”, to become: “If any number of weights keep a balance in the horizontal position, if any two weights are moved along the balance in opposite directions of the same amount, the balance still remains in the horizontal position”.
Although this work has the main purpose of illustrating the way in which mathematics was used in ancient Greek mechanics, we believe it appropriate to make some considerations on the merits of the treatment of the law of the lever. In the interpretation provided above of the Mechanica Problemata, one starts from the idea that a force is the more effective the faster the movement of its point of application. In other words, to establish whether the lever is in a state of rest, reference is made to its possible motion, that is, to its kinematics. The formulations of the Hellenistic mathematicians concerning mechanics, of Euclid and Archimedes in primis, do not refer to kinematics; for this, they are labeled as purely static approaches.
The power acquires forces from the length of the arm and therefore from the consequent speed; in fact, the longer the arms, the more they are fast at their ends [.].This assertion is certainly true and abundantly substantiated. But we do not agree that the cause of this wonderful effect is the speed resulting from the length of the arm. What speed in fact can an immobile thing have? In fact, the lever and the balance are immobile as long as they are kept in balance and nothing but a small one power bears great weight.[23] (p. 126)