Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. It may be thought of as the opposite of “slipperiness”.

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 between non-moving surfaces, and kinetic friction between moving surfaces.

* Fluid friction describes the friction between layers within a viscous fluid that are moving relative to each other.[1][2]

* Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces.[3][4][5]

* Skin friction is a component of drag, the force resisting the motion of a solid body through a fluid.

* Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.[2]

via Friction – Wikipedia, the free encyclopedia.