section_path: Examples of forces in classical mechanics/Friction
section_name: Friction
paragraph_text: Friction
  
  Friction is a force that opposes relative motion of two bodies. At the macroscopic scale, the frictional force is directly related to the normal force at the point of contact. There are two broad classifications of frictional forces: static friction and kinetic friction.
  
  The static friction force (F s f {\displaystyle \mathbf {F} _{\mathrm {sf} }}) will exactly oppose forces applied to an object parallel to a surface up to the limit specified by the coefficient of static friction (μ s f {\displaystyle \mu _{\mathrm {sf} }}) multiplied by the normal force (F N {\displaystyle \mathbf {F} _{\text{N}}}). In other words, the magnitude of the static friction force satisfies the inequality: 0 ≤ F s f ≤ μ s f F N. {\displaystyle 0\leq \mathbf {F} _{\mathrm {sf} }\leq \mu _{\mathrm {sf} }\mathbf {F} _{\mathrm {N} }.}
  
  The kinetic friction force (F k f {\displaystyle F_{\mathrm {kf} }}) is typically independent of both the forces applied and the movement of the object. Thus, the magnitude of the force equals: F k f = μ k f F N, {\displaystyle \mathbf {F} _{\mathrm {kf} }=\mu _{\mathrm {kf} }\mathbf {F} _{\mathrm {N} },}
  
  where μ k f {\displaystyle \mu _{\mathrm {kf} }} is the coefficient of kinetic friction. The coefficient of kinetic friction is normally less than the coefficient of static friction.