The Physics
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Opus in profectus

Friction

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Discussion

The force between surfaces in contact that resists their relative tangential motion (slipping).

Types: static & kinetic

Classical Approximations

f ≤ μN

fs ≤ μsN

fk = μkN

microscopic description

miscellaneous stuff

Coefficients of friction for selected interfaces (in order of generally decreasing value)
μs μk interface 
  1.16 rubber - rubber
  1.02 rubber - concrete
0.84–0.98 0.72 car tire - asphalt
  0.35 car tire - grass
0.78–1.00   skin - metals
0.9–1.0   glass - glass
  0.9 sheep - steel mesh
  0.7 sheep - plastic batten (⊥)
  0.6 sheep - plastic batten (∥)
  0.6 sheep - wood batten (⊥)
  0.5 sheep - wood batten (∥)
0.68 0.67 horseshoe - rubber
0.48 0.47 horseshoe - concrete
0.44 0.42 horseshoe - asphalt
0.42 0.38 horseshoe - cobblestone
0.58   steel - steel
0.40   brake pads - cast iron
0.30–0.70   iron - stone
0.62   wood - concrete
0.6   wood - brick
0.40   wood - stone
0.2–0.6   wood - metals
0.29 0.22 wood - felt
0.28 0.17 wood - wood
0.56   leather - metals
0.27–0.38   leather - wood
  0.225 tangerine peel - linoleum floor
  0.125 apple peel - linoleum floor
  0.066 banana peel - linoleum floor
0.3   snow - nylon
0.04–0.4 0.04–0.4 snow - hickory, waxed
0.1   graphite - graphite
0.1   graphite - steel
0.05–0.08   teflon - steel
  0.32–0.36 teflon - teflon (189 cm/s)
  0.05–0.08 teflon - teflon (1.1 cm/s)
  0.03 ice - steel
0.05–0.5 0.02–0.09 ice - ice
  0.00440.0057 cartillage - synovial fluid
  0.0013 tendon - sheath