Static friction

Static friction

We speak of static friction when the surfaces are at rest relative to each other, and a force exists on the object to set it in motion.

The force of static friction opposes the force which would cause the object to move: up to a limit, it is equal in magnitude but opposite in direction to the applied force:

\[\mathbf{F}_{\mathrm{s}} = - \mathbf{F}_{\mathrm{applied}} \qquad{} \mathrm{if}\ \left|\mathbf{F}_{\mathrm{applied}}\right| \leq F_{\mathrm{s, max}}.\]

The surface cannot resist an applied force of any magnitude: if the magnitude of the applied force is greater than the maximum value of the force of static friction, the object starts to slide and friction turns into sliding friction.

The maximum value of the force of static friction depends on the magnitude of the normal force between the object and the surface and on the roughness of the surfaces:
\[F_{\mathrm{s, max}} = \mu_0 F_{\mathrm{N}}, \qquad \mathrm{so}\quad F_{\mathrm{s}} \leq \mu_0 F_{\mathrm{N}},\]

where \(\mu_0\) is called the coefficient of static friction. It is different to the coefficient of kinetic friction \(\mu\): usually \(\mu_0 > \mu\) for two given surfaces.

The following figure summarises the relations between the different types of friction and the applied force:

Licensed under the Creative Commons Attribution Share Alike License 4.0