Define angle of friction and show that tan θ =μ, where θ is angle of friction and μ is coefficient of friction.

Angle of friction: It is the angle which the resultant of force of friction and normal reaction make with the normal reaction.
Let a body of weight W be placed on the horizontal surface and pulled by force F. Since weight of the body is acting in vertically downward direction and there is no component of force F in vertical direction, therefore, normal reaction will be equal to weight. Let μ be coefficient of friction. Therefore, force of friction f = μW will act in horizontal direction opposite to the direction of F applied.

Let the resultant of  make an angle  with 
∴                        
or