The Painleve paradoxes appear when considering this phenomenon for rigid bodies. Along with gravity and particle shape, the angle of repose of granular materials depends on the arctangent of this value in a situation to that 
The Painleve paradoxes appear when considering this phenomenon for rigid bodies. Along with gravity and particle shape, the angle of repose of granular materials depends on the arctangent of this value in a situation to that of a block on an inclined plane. Common approximations for this phenomenon state that it is independent of contact area and relative velocity, so a simple equation for this force sets it equal to mu times the normal force, and that coefficient may be static or kinetic. For 10 points, name this force that opposes motions.
The Painleve paradoxes appear when considering this phenomenon for rigid bodies.