On Friction: Contact Area and the Wearing Compass Point
Equal weights give equal starting resistance whatever the contact shape; broader contact endures longer.
Continuing the friction discussion in mirror-script, this page opens by observing that the end of a circle never truly rejoins its beginning, the traced line drifting an imperceptible amount toward the center. Leonardo then states that a single weight offers equal resistance at the start of its motion regardless of the width or length of contact. He adds a rule that the larger the contact surface of dense rubbing bodies, the longer the friction endures, and the smaller it is the sooner it wears, illustrating with the head of a knife's handle lasting longer than its point.
On this page
Contact size governs how long friction lasts
Leonardo asserts that friction from one and the same weight gives equal resistance at the start of motion whatever the width or length of the contact. He then holds that the greater the area of contact between dense bodies, the longer the friction persists, and the smaller it is the less durable it is. He grounds this in the head of a knife-handle being more preserved over equal time than its point.
The circle whose end never rejoins its start
The opening lines reason that the terminus of a traced circular line does not join its beginning but approaches, by some imperceptible amount, toward the center of the circle. The remark ties the geometry of drawing a perfect circle to the physical wearing of the tracing instrument. It sets up the argument continued on the following folio.