Friction angle
= Friction angle
{title2=$\varphi=\arctan\mu$}
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 $\tan\varphi=|F_t|/N$, 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.