Figure 1: Simulated blocks with fractal rough surfaces, exhibiting static<unk> al interactions<unk>  is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of<unk> : *Dry<unk>  is a force that opposes the relative lateral motion of two solid surfaces in contact. Dry<unk>  is subdivided into static<unk>  ("stiction") between non-moving surfaces, and kinetic<unk>  between moving surfaces. With the exception of atomic or molecular<unk> , dry<unk>  generally arises from the interaction of surface features, known as asperities (see Figure 1). *Fluid<unk>  describes the<unk>  between layers of a viscous fluid that are moving relative to each other. *Lubricated<unk>  is a case of fluid<unk>  where a lubricant fluid separates two solid surfaces. *Skin<unk>  is a component of drag, the force resisting the motion of a fluid across the surface of a body. *Internal<unk>  is 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<unk>  between the two surfaces converts kinetic energy into thermal energy (that is, it converts work to heat). This property can have dramatic consequences, as illustrated by the use of<unk>  created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to thermal energy whenever motion with<unk>  occurs, for example when a viscous fluid is stirred. Another important consequence of many types of<unk>  can be wear, which may lead to performance degradation or damage to components.<unk>  is a component of the science of tribology.<unk>  is desirable and important in supplying traction to facilitate motion on land. Most land vehicles rely on<unk>  for acceleration, deceleration and changing direction. Sudden reductions in traction can cause loss of control and accidents.<unk>  is not itself a fundamental force. Dry<unk>  arises from a combination of inter-surface adhesion, surface roughness, surface deformation, and surface contamination. The complexity of these interactions makes the calculation of<unk>  from first principles impractical and necessitates the use of empirical methods for analysis and the development of theory.<unk>  is a non-conservative force – work done against<unk>  is path dependent. In the presence of<unk> , some kinetic energy is always transformed to thermal energy, so mechanical energy is not conserved.
Friction
Overpressure
Solvent drag
Adhesion 
Fixation 
Subluxation
Effacement 
Disarticulation