Onset of Sliding Friction and Oblique Pulling
Pulley experiments on when a load begins to move and how oblique pull raises its friction
A column of small pulley diagrams pits a load (marked 8) against a smaller force (marked 1) to study friction (confregatione). Leonardo notes that motion begins only when points such as S p c or t x u fall into a straight line, and that the ratio of certain lines gives the ratio of force to natural weight, i.e. the true friction. In the lowest figure he reasons that an 8-pound stone needs one pound of pull, but pulling obliquely (line a n losing a third of n m) adds 1/3, so the mover feels one pound and 1/3.
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Motion begins only when the points fall in a straight line
No motion occurs unless b first descends to a, so that m b n becomes the straight line m a n; the ratio of line o 8 to line 8 n then equals that of force to natural weight, the true friction. Likewise motion occurs when t x u align, x entering the place of u.
Labor of the mover versus the force added to it
The proportion that r S has with r t equals that which the true labor of the mover has with the force added to it.
Oblique pull raises the friction to one pound and a third
The ratio of a n to n m gives the added difficulty of pulling obliquely by the body's own weight. An 8-pound stone needs one pound of pull; pulling obliquely, line a n loses a third of n m and so gains 1/3 of the friction, so the mover feels one pound and 1/3.