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 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.