The angle made by the resultant of normal reaction and limiting value of frictional force with normal to the surface is called

Here, $\mathrm{Rcos\theta }=\mathrm{N}$,$\mathrm{Rsin\theta }=\mathrm{f}$

$\mathrm{tan\theta }=\frac{\mathrm{f}}{\mathrm{N}}=\mathrm{\mu }$

So, $\mathrm{\theta }$ is angle of friction.

The angle between the resultant contact force and the normal force exerted by a body on the other is called the angle of friction.