Friction is the forceresisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *.Dry frictionresists relative lateral motion of two solid surfacesin contact. Dry friction is subdivided intostatic friction(" stiction") between non-moving surfaces, andkinetic frictionbetween moving surfaces. *.Fluid frictiondescribes the friction between layers of a viscousfluid that are moving relative to each other. *.Lubricated frictionis a case of fluid friction where a lubricantfluid 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 frictionis 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 energyinto thermal energy.