Static Friction

Static friction, abbreviated as \(f_s\), is the force that opposes the relative motion or impending motion of two surfaces in contact that are not moving with respect to each other.  Static friction happens between zero and the smallest force needed to start the motion of an object.  In order for an object to move the static friction must be overcome, when this happens you experience kinetic friction.   Static friction occurs because the irregularities on the surfaces in contact interlock with each other, creating a resistance to motion.  The strength of static friction depends on the materials in contact, the force pressing the two surfaces together, and the roughness of the surfaces.

The maximum kinetic friction force that can be exerted between two surfaces is proportional to the normal force, which is the force perpendicular to the surfaces that is pressing them together.  If the applied force is greater than the maximum static friction force, the two surfaces will begin to move with respect to each other and kinetic friction will take over.

Laws of Static Friction

The maximum static friction force is proportional to the normal force -  The maximum static friction force that can be exerted between two surfaces is proportional to the normal force, which is the force perpendicular to the surfaces that is pressing them together.
The direction of the static friction force is opposite to the applied force  -  The direction of the static friction force is always opposite to the direction of the applied force.  This means that the static friction force is exerted in a direction that opposes the tendency of the surfaces to move with respect to each other.  If the applied force is increased beyond the maximum static friction force, the surfaces will begin to move and kinetic friction will take over.

 

Static friction formula
\( f_s =   \mu_s \; F_n \)     (Static Friction) \( \mu_s =   f_s \;/\; F_n \) \( F_n =  f_s \;/\; \mu_s \)
Symbol	English	Metric
\( f_s \) = Static Friction	\( lbf \)	\(N\)
\( \mu_s \)  (Greek symbol mu) = Static Friction Coefficient	\( dimensionless \)	\( dimensionless \)
\( F_n \) = Normal Force	\( lbf \)	\(N\)