Mechanics

Friction

Friction results from the complex interaction between two systems in contact. It is both an essential force in the propulsion of land vehicles and a force that dissipates some of the small power output provided by the human machine.

From: "Rolling Bearings and their Contribution to Progress in Technology,"
FAG Kugelfischer (1986)

This problem has been present from historical time. The diagram above is from the tomb of Tehuti-Hetep in circa 1880 BCE, and shows an Egyptian statue with a weight, W, of about 600 kN being moved by 172 men on a sled running on wooden planks placed on the ground. To minimize the friction interaction between the sled and the planks, water is being poured at the front end of the sled (blue line) to form a lubricating layer. Information in the picture may be used to estimate the friction coefficient of the wet wood surfaces. If each of the men could pull on the ropes with a force of 800 N, their total force must just overcome that due to friction if the sled is to move. A force, F, of 138 kN is, therefore, a maximum value for the friction interaction . Since F = mW, this estimate gives a friction coefficient, m, of 0.23 - a good value for wet wood!