The **static friction** force represents the force that must be overcome in order to set bodies in motion, and is defined by the formula:

\[F_s=\mu_s\cdot F_{\perp}\]

wherewith (F_{perp}) indicates the force that acts perpendicularly against the contact surface of the two bodies and with (mu_s) the static friction coefficient, which depends on the materials and surfaces involved and which has no unit of measurement (it is a pure number).

Very often the perpendicular force \(F_{\perp}\) coincides with the weight force, but it is not always so. More generally, the component perpendicular to the surface of the resultant of the forces applied to the body must be considered. For example, imagine pressing down a block on a table with one hand downwards; in this case the component perpendicular to the surface is given by the sum of the weight force and the force exerted by the hand.