 Friction angle

The limiting angle between a contact resultant and the compressive normal force when static friction obeys $|F_t|\le \mu N$. Resolving the resultant gives $\tan \phi =|F_t|/N$, so admissible contact forces lie within a cone of half-angle $\arctan \mu$ about the inward normal. A contact-force magnitude is generally $\sqrt{N^2+F_t^2}$, distinct from N.