what is angle of friction?

It is the minimum angle at which a body on an incline would start sliding down the incline.(i.e overcome the friction and start sliding)

TanØ=µ_s

Angle of friction

For the maximum angle of static friction between granular materials, see Angle of repose.

For certain applications it is more useful to define static friction in terms of the maximum angle before which one of the items will begin sliding. This is called the angle of friction or friction angle. It is defined as:

where θ is the angle from vertical and µ_s is the static coefficient of friction between the objects.^[24] This formula can also be used to calculate µ_s from empirical measurements of the friction angle.

at the point of rough contact, where slipping is about to occur the two forces act on object i.e. normal reaction N and frictional force μN. The resultant of these two forces is F and it makes an angle Θ with the normal rectn force....here Θ is angle of friction

tan Θ = μN/N

        = μ

Angle of friction is the angle between the resultant vector(resultant vector:-resultant vector of   frictional force and normalforce) and normal force vector.

angle of friction is the angle between the resultant of normal and friction and the normal.