For 10 points, name
 this force that retards the sliding of two surfaces across each other.
For a solid-liquid suspension, Ralph Bagnold defined a "dynamic" version of this force and
 characterized its "effective angle" as the arctangent of the ratio of the normal and shear stresses. The
 Bilman-Sorine Model modifies the Dahl Model to account for this force's decrease with increasing velocity
 in the low-velocity regime, an effect discovered by Richard Stribeck. Counterintuitively, it is independent
 of cross-sectional area by Amontons' Second Law. Amontons' First Law, rediscovered by Charles
 Coulomb, states that the magnitude of this force is proportional to the normal force. For 10 points, name
 this force that retards the sliding of two surfaces across each other.
For a solid-liquid suspension, Ralph Bagnold defined a "dynamic" version of this force and
 characterized its "effective angle" as the arctangent of the ratio of the normal and shear stresses.
friction