The ASTM G 2 Committee on Wear and Erosion maintains a list of definitions for friction, wear, and erosion terms: ASTM G 40. Unfortunately, this list of terms does not include friction. It defines friction-related terms: friction force, coefficient of friction, kinetic coefficient of friction, static coefficient of friction, and stiction. The dictionary definitions of friction range from “rubbing” to “the resistance that a body encounters when movement on another body is attempted or sustained.” The terms rubbing and resistance are included in the majority of the dictionary definitions. However, friction is encountered in fluids even between molecules rubbing in a solid. For example, plastics can locally heat when they are repeatedly stressed as in a fatigue test. Also, it is well known that friction of ship hulls versus water is a significant factor for fuel usage. A proper definition of friction should include all of these types of friction, all of those shown in Figure 11.1:
Sliding-solids rubbing Rolling-a solid rolling on a solid surface Lubricated-sliding or rolling with lubricant in the contact Internal-within a solid Atomic-between the atoms of contacting surfaces
An inclusive definition might be the force resisting the start of motion or continued motion of one body or substance on or within another body or substance. The problem with this definition is that friction is also inextricably connected with energy dissipation. As mentioned, some materials like automobile tires get hot from flexing. This heat must be dissipated. A significant amount of energy, heat, must be dissipated in braking a vehicle or moving part on a machine. Thus, rubbing solids generate heat. Forces resisting motion generate heat. And this is logical since the forces occur in sliding and sliding involves relative motion or distance, and we know that a force times a distance yields an energy term. However, attempts to add an energy term to a friction definition have been met with dissension among the tribology researchers. So, suffice it to say at this point that friction is a resisting force that must be overcome to start or to sustain motion.