Angle of Friction

The angle of friction is defined geometrically: it is the angle $\theta$ between the normal force $N$ and the resultant $R$ of the normal force and the maximum static friction force $f_{s,max}$ (i.e. $\vec R=\vec N+\vec f_{s,max}$).

From the right-triangle geometry:

But since, by the friction law, sliding begins exactly when $f_{s,max}=\mu_sN$:

$\tan\theta=\mu_s.$ …