Archimedes on statics

In On the Equilibrium of Planes, Archimedes discusses statics and the law of the lever in Book I, Postulates and Propositions. In the following extract Archimedes writes:-

I postulate the following:

1. Equal weights at equal distances are in equilibrium, and equal weights at unequal distances are not in equilibrium but incline towards the weight that is at the greater distance.

2. If, when weights at certain distances are in equilibrium, something be added to one of the weights, they are not in equilibrium, but incline towards that weight to which the addition was made.

3. Similarly, if anything be taken away from one of the weights, the are not in equilibrium but incline towards the weight from which nothing was taken.

4. When equal and similar plane figures coincide if applied to one another, their centres of gravity similarly coincide.

5. In figures which are unequal but similar the centres of gravity will be similarly situated. By points similarly situated in relation to similar figures I mean points such that, if straight lines be drawn from them to the equal angles, they make equal angles with the corresponding sides.

6. If magnitudes at certain distances be in equilibrium, (other) magnitudes equal to them will also be in equilibrium at the same distances.

7. In any figure whose perimeter is concave in (one and) the same direction the centre of gravity must be within the figure.

Proposition 1. Weights which balance at equal distances are equal ....

Proposition 2. Unequal weights at equal distances will not balance but will incline towards the greater weight ....

Proposition 3. Unequal weights will balance at unequal distances, the greater weight being at the lesser distance ....

Proposition 4. If two equal weights have not the same centre of gravity, the centre of gravity of both taken together is at the middle point of the line joining their centres of gravity ....

Proposition 5. If three equal magnitudes have their centres of gravity on a straight line at equal distances, the centre of gravity of the system will coincide with that of the middle magnitude ....

Corollary 1. The same is true of any odd number of magnitudes if those that are at equal distances from the middle one are equal, while the distances between their centres of gravity are equal.

Corollary 2. If there be an even number of magnitudes with their centres of gravity situated at equal distances on one straight line, and if the two middle ones be equal, while those which are equidistant from them (on each side) are equal respectively, the centre of gravity of the system is the middle point of the line joining the centres of gravity of the two middle ones.

Propositions 6, 7. Two magnitudes, whether commensurable [Proposition 6] or incommensurable [Proposition 7], balance at distances reciprocally proportional to the magnitudes.

JOC/EFR August 2006

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