When two solid surfaces are brought into contact, a finite normal force is needed to pull the two solids apart. This force is known as the force of adhesion or simply adhesion.
The coefficient of adhesion () defined as the ratio of tractive effort required to propel the wheel of a locomotive to its adhesive weight. Adhesion traction is the friction between the drive wheels and the steel rail.
µ = Ft/W
Where
Ft = Tractive effort
W = Adhesive Weight
EXAMIANS