The coefficient of friction between any two surfaces in contact is defined as the ratio of the force of limiting friction and normal reaction between them.
Consider a block of mass m moving on the ground. Let F indicate the limiting frictional force. The limiting friction force acts in a direction opposing the motion of the block. Let the normal reaction R make an angle \(\theta\) with the resultant of F and R.
\(tan \theta = \frac{DC}{OD} = \frac{OA}{OD} = \mu = \frac{F}{R}\)
Hence, µ = tan \(\theta\)
