<unk>  is the force resisting the relative motion of two surfaces in contact or a surface in contact with a fluid (e.g. air on an aircraft or water in a pipe). It is not a fundamental force, as it is derived from electromagnetic forces between atoms and electrons, and so cannot be calculated from first principles, but instead must be found empirically. When contacting surfaces move relative to each other, the<unk>  between the two objects converts kinetic energy into thermal energy, or heat.<unk>  between solid objects is often referred to as dry<unk>  or sliding<unk>  and between a solid and a gas or liquid as fluid<unk> . Both of these types of<unk>  are called kinetic<unk> . Contrary to popular credibility, sliding<unk>  is not caused by surface roughness, but by chemical bonding between the surfaces. Surface roughness and contact area, however, do affect sliding<unk>  for micro- and nano-scale objects where surface area forces dominate inertial forces. Internal<unk>  is the motion- resisting force between the surfaces of the particles making up the substance.
Friction
Adhesion 
Subluxation
Stacking 
Wasting
Swelling 
Crush injury
Extraction 