# Changes

## Friction

, 12:13, 24 June 2009
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'''Friction''' (sometimes called '''sliding friction''' to distinguish from rolling friction) is the component of [[contact force]] between two bodies with a surface of contact acting in a direction along (or parallel to) the surface of contact, or the tangent plane through the point of contact.

The term '''friction''' refers by default to '''dry friction''': friction where the two surfaces of contact are dry and solid. The typical model used for such friction is the [[Coulomb model of friction]].
===Tendency-based definition===
===Types of friction===
* [[Static friction]]: This is friction exerted when the surfaces of contact are not slipping relative to each other. This occurs, for instance, both when the surfaces are stationary, and in the case of [[rolling]]. Static friction acts to precisely cancel the net external force along the plane of contact. The According to the [[Coulomb model of friction]], the maximum possible value of static friction is $\mu_sN$ where $N$ is the [[normal force]] and $\mu_s$ is the [[limiting coefficient of static friction]] for the two surfaces.* [[Kinetic friction]] or dynamic friction: This is friction exerted when the surfaces are slipping relative to each other. Kinetic friction acts in the direction opposite to the direction of slipping. According to the [[Coulomb model of friction]], and for small relative velocities, its magnitude is $\mu_kN$, where $N$ is the normal force and $\mu_k$ is the [[coefficient of kinetic friction]].
===Forces between rigid bodies===