Summarizing Friction

Summary

• Friction is a contact force between systems that opposes the motion or attempted motion between them. Simple friction is proportional to the normal force \(N\) pushing the systems together. (A normal force is always perpendicular to the contact surface between systems.) Friction depends on both of the materials involved. The magnitude of static friction\({f}_{\text{s}}\) between systems stationary relative to one another is given by
  \({f}_{\text{s}}\le {\mu }_{\text{s}}N,\)
  where \({\mu }_{\text{s}}\) is the coefficient of static friction, which depends on both of the materials.
• The kinetic friction force \({f}_{\text{k}}\) between systems moving relative to one another is given by
  \({f}_{\text{k}}={\mu }_{\text{k}}N,\)
  where \({\mu }_{\text{k}}\) is the coefficient of kinetic friction, which also depends on both materials.

Glossary

friction

a force that opposes relative motion or attempts at motion between systems in contact

kinetic friction

a force that opposes the motion of two systems that are in contact and moving relative to one another

static friction

a force that opposes the motion of two systems that are in contact and are not moving relative to one another

magnitude of static friction

\({f}_{s}\le {\mu }_{s}\phantom{\rule{0.25em}{0ex}}N\), where \({\mu }_{s}\) is the coefficient of static friction and \(N\) is the magnitude of the normal force

magnitude of kinetic friction

\({f}_{k}={\mu }_{k}N\), where \({\mu }_{k}\) is the coefficient of kinetic friction