On weight: the mechanics of pulleys and tackle
How a moving load is felt at the fixed point, and the laws of block-and-tackle
Headed 'On weight', this leaf studies pulleys and tackle with diagrams lettered a, b, c and d and marked 12, 8, 4, 16 and 80. Leonardo argues that a load of twelve held immobile is felt in full at the support c, but when it is in motion c feels only eight, and reasons through the 'quaternaries' that make up that eight to show the remainder descends free. A further case shows that when the upward motion of b is blocked, c feels sixteen, and closing rules state that the greatest power of a tackle lies in the cord joined to its motor, that the weight divides equally among the cords, and that the cord nearest the motor runs fastest.
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A load of 12 felt as 8 while in motion
With a b at twelve and immobile, the support c feels the whole twelve; but once a b is in motion c feels only eight, and the excess of four has no support yet would not fall. Leonardo reasons through the two 'quaternaries' composing eight to conclude that the four in balance b has neither gravity nor levity, so the remaining four of a descends freely against only the air and the friction of the balance's pivot.
When the rise of b is blocked, c feels 16
In a second case the upward motion of b is forbidden; then the support c will feel sixteen, since the eight at a is resisted by no more than eight, and even were the whole weight at b the balance c would never feel more than sixteen.
Laws of block-and-tackle
General rules follow: the greatest power of a tackle is in the cord joined to its motor and the least in the cord joined to one of the pulleys; the weight divides equally among the cords running over the pulleys; and the cord nearest the motor is the fastest, while the most remote is the slowest.