specific type of force that resists the relative motion between two objects in contact. The direction is always opposite the motion.

Merriam-Webster Online Dictionary
friction (noun)
a) the rubbing of one body against another
b) the force that resists relative motion between two bodies in contact
the clashing between two persons or parties of opposed views - disagreement
sound produced by the movement of air through a narrow constriction in the mouth or glottis
friction (Wikipedia)
For other uses, see Friction (disambiguation).

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:

  • Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction ("stiction") between non-moving surfaces, and kinetic friction between moving surfaces.
  • Fluid friction describes the friction between layers of a viscous fluid that are moving relative to each other.
  • Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid surfaces.
  • Skin friction is a component of drag, the force resisting the motion of a fluid across the surface of a body.
  • Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.

When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy (that is, it converts work to heat). This property can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to thermal energy whenever motion with friction occurs, for example when a viscous fluid is stirred. Another important consequence of many types of friction can be wear, which may lead to performance degradation and/or damage to components. Friction is a component of the science of tribology.

Friction is not itself a fundamental force. Dry friction arises from a combination of inter-surface adhesion, surface roughness, surface deformation, and surface contamination. The complexity of these interactions makes the calculation of friction from first principles impractical and necessitates the use of empirical methods for analysis and the development of theory.

Friction is a non-conservative force - work done against friction is path dependent. In the presence of friction, some energy is always lost in the form of heat. Thus mechanical energy is not conserved.

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