Angle of friction

Definition

Given two surfaces, the angle of repose between the two surfaces is defined as the angle $\theta$ that a fixed inclined plane made up of one surface must make with the horizontal so that a body with the second surfaces placed on it just starts slipping.

This angle is numerically given by:

$\!\theta =\tan ^{-1}(\mu _{s})$

where $\mu _{s}$ is the limiting coefficient of static friction.

It is numerically equal to the angle of friction.