As in all fields of measurement, one is free to choose a system of units. Thus although the furlong-fortnight-stone system of units may be a convenient system of units for horse racing, and the foot-slug-second system may suit engineering, they are inconvenient for scientific measurements. The meter-kilogram-second system of units (M.K.S.) is the system that has been adopted world wide as the international system of units in science. The other metric system is the centimeter-gram-second system which was commonly used before adoption of the MKS system. The United States is one of the only countries still using the archaic foot-pound-second and foot-slug-second systems which are commonly called the English systems. These archaic units were abandoned decades ago in England. In this course, we will adopt the rationalized MKS system of units.