Published: Jan 2007
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MANY CLEVER PEOPLE HAVE BUILT PERPETUAL motion machines and no one has succeeded in making one that works. The machines eventually stop because of friction. Friction is an energy dissipation process that accompanies a body or substance in relative motion in contact with another body or substance. When a solid body slides on another solid, it takes energy to put the body in motion and keep it moving. This energy is dissipated at the rubbing surfaces, usually in the form of heat or deformation. When a solid body is moved through a fluid, the energy required to produce motion is dissipated by movement of the fluid as in waves or even results in heating. Re-entry of the U.S. space shuttle is an example of friction between a solid (the shuttle) and a fluid (air) and the energy of the collision and rubbing of air molecules on the shuttle surfaces is heating. A fluid in a pipe dissipates energy at the fluid/pipe interface. Rolling a revolute shape like a tire on a solid surface requires energy and this energy is dissipated by tire and roadway heating. There is no zero friction mechanism because there is always rubbing when a body or substance is set in motion and rubbing of solid surfaces or substances dissipates energy. A first principle of physics is “for any force there is an equal and opposite reaction force.” The friction force is often the reaction force dictated by physics. The mathematical definition of friction force, per ASTM G 40, is: F - the resisting force tangential to the interface between two bodies when, under the action of an external force, one body moves or tends to move relative to the other.
Amonton in the 18th century produced the formula for a unitless quantity to quantify the friction force, the coefficient of friction (µ). The ASTM G 40 definition is: In tribology, the dimensionless ratio of the friction force (F) between two bodies to the normal force (N) pressing the bodies together: µ = F/N.
In contacting solids, it is universally agreed that the force to initiate motion of a body can be different from the force required to sustain relative motion of one body on another, so two coefficients of friction have evolved; the static and kinetic coefficients of friction. The static coefficient of friction is the coefficient of friction corresponding to the maximum friction force that must be overcome to initiate macroscopic motion between two bodies (per ASTM G 40). The kinetic coefficient of friction is the coefficient of friction corresponding to the friction force needed to sustain relative motion between two bodies in contact at any point in time.