When two objects are in rough contact and friction is limiting, two contact forces act on each object; one is the normal reaction R and the other is the frictional force uR.

 

 

 

 

 

 

 

 

 

 

 

 

The resultant of R and uR, which is sometimes called the resultant contact force, is shown in the diagram.

Its magnitude Ö(R^2 + [mR]^2) = RÖ(1 + m^2)

The angle between this resultant and the normal reaction is called the angle of friction and is denoted by  l where

          Tan l = F/R = mR/R

i.e. l = m

The tangent of the angle of friction is equal to the coefficient of friction

There are occasions when it is convenient to use the resultant contact force.