When its strength alternates with motion, the stick-slip phenomenon occurs.  
The Sommerfeld number is important in the study of decreasing this phenomenon, and the Frenkel-Kontorova-Tomlinson model describes it.  Its coefficient is related to the Stribeck number by the Stribeck curve.  It causes the formation of tribofilms.  It is described by a pair of laws named for Amonton, and the point where one form of it is overcome is the angle of repose. This force is independent of sliding velocity and proportional to the normal force.  When its strength alternates with motion, the stick-slip phenomenon occurs.  For 10 points, name this phenomenon, which represents the force resisting any sliding motion, and which comes in static and kinetic varieties.
The Sommerfeld number is important in the study of decreasing this phenomenon, and the Frenkel-Kontorova-Tomlinson model describes it.  
friction