Static friction describes the friction force
opposing the direction of motion when the
sliding velocity is zero, static friction is
defined as:
F = μR
Where,
F = Friction force
μ = Coefficient of Friction (COF)
R = |W| = Reaction force due to the
applied load W
Note the applied load is assumed to be
given by the mass above the contact area only (active area, coloured green in the diagram).