ABSTRACT
Designers need equations for all phenomena that control the cost, function, and reliability of the products they design. There are useful equations for fluid flow through pipes, for the energy required to achieve mechanical action, and for the voltage drop across a resistor. But there are no broad and "user friendly" equations for predicting the frictional behavior, the adequacy of lubrication under severe contact conditions, or the wear life of mechanical products. However, many equations contain variables that are not readily definable, or are only available from experiment. Three major types of equations is discussed: fundamental, empirical, and semiempirical. Fundamental equations are based on a known singular physical mechanism. Empirical equations are those developed from experimental data by fitting curves to the plotted data or by estimating equations using other statistical methods. Mathematical models are equations that simulate, or describe the response of, some entity of unknown internal composition ("black box") to some input variable.
