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.
* Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces.
* 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.