Friction is the force resisting the relative lateral (tangential) motion of solid surfaces, fluid layers, or material elements in contact. It is usually subdivided into several varieties: * Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is also subdivided into static friction between non-moving surfaces, and kinetic friction (sometimes called sliding friction or dynamic friction) between moving surfaces. * Lubricated friction or fluid friction resists relative lateral motion of two solid surfaces separated by a layer of gas or liquid. * Fluid friction is also used to describe the friction between layers within a fluid that are moving relative to each other. * Skin friction is a component of drag, the force resisting the motion of a