Angle of friction is the
The angle of friction (?):
It is defined as the angle made by the resultant reaction (S) with normal reaction (R) and limiting friction.
The resistance offered to the motion is known as friction when one surface slides or tends to slide over another surface. It is also called a force of friction or frictional resistance.
Coefficient of friction (μ):
It is the ratio of the magnitude of the limiting force of friction to the normal reaction between surfaces in contact