The component of the contact force from a surface that is parallel to the plane of the surface. Friction forces will arise when (1.) an object is sliding along the surface or (2.) an object resting on the surface is subject to forces that would cause it to slide in the absence of friction. The force of friction will always resist the existing or intended sliding motion.

Motivation for Concept

It takes effort to get an object sliding along a surface, and sustained effort to keep the object moving once started. The effort required will depend on the surface characteristics and the object's characteristics. Sliding a wooden block along a tiled floor is much easier than sliding it along a rubber mat. The effort required will also depend upon the contact force between the object and the surface. Brushing sandpaper lightly across wood is easy, but when the sandpaper is pressed hard against the wood, movement requires substantial effort.

Static versus Kinetic Friction

Friction has two basic manifestations that are qualitatively different.

  • Static Friction: The specific manifestation of friction which attempts to resist efforts to move an object that is currently at rest with respect to a surface. If possible, static friction provides just enough force to keep the object stationary, and no more. When the net force attempting to create sliding motion exceeds a certain limiting value proportional to the normal force exerted by the surface on the object, static friction will be unable to prevent motion.
  • Kinetic Friction: The specific manifestation of friction that is directly opposed to an object's sliding motion along a surface. The force of kinetic friction has a size independent of the speed of the object, and proportional to the normal force exerted on the object by the surface.