# Static Friction

Written by Jerry Ratzlaff on . Posted in Classical Mechanics

Static friction, abbreviated as $$f_s$$, is the force that resists relative movement and keeps objects at rest.  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

## Laws of Static Friction

• The force of friction that is trying to oppose the movement of a stationary object.
• The force of friction always acts in a direction opposite to that in which the body tends to move.
• The magnitude of the force of friction is equal to the force which tends the body to move.
• The force of friction is independent of the area of contact between the two surfaces.
• The force of friction depends upon the roughness of the surfaces.

## Static friction formula

 $$\large{ f_s = \mu_s \; F_n }$$

### Where:

 Units English Metric $$\large{ f_s }$$ = static friction $$\large{ lbf }$$ $$\large{N}$$ $$\large{ F_n }$$ = normal force $$\large{ lbf }$$ $$\large{N}$$ $$\large{ \mu_s }$$  (Greek symbol mu) = static friction coefficient $$\large{ dimensionless }$$