9.1 Angle of friction $\left( \lambda \right)$

1.1 Inertia
1.2 Momentum

2.1 Equations of Newton's second law of motion

4.1 Types of force

7.1 Pulley
7.2 Constrained motion

9.1 Angle of friction $\left( \lambda \right)$
9.2 Angle of repose $\left( \alpha \right)$

The angle of friction between any two surfaces in contact is defined as the angle which the resultant of the force of limiting friction $F$ and normal reaction $N$ makes with the direction of normal reaction $(N)$.

Angle of friction is represented by $\lambda $.

The resultant of these two forces is $F$ and it makes an angle $\lambda $ with the normal where, $$\begin{equation} \begin{aligned} \tan \lambda = \frac{{\mu N}}{N} \\ \tan \lambda = \mu \\ \lambda = {\tan ^{ - 1}}\mu \\\end{aligned} \end{equation} $$