Speed's the rate of motion, or equivalently the rate of change of distance. Speed's a scalar quantity with dimensions length/time; the equivalent vector quantity to speed's velocity. Speed's measured in the same physical units of measurement as velocity, but doesn't contain the element of direction that velocity has. Speed's thus the magnitude component of velocity. In mathematical notation, if an object traveling at constant speed moves a distance x in time t, its speed, denoted by v,'s simply… (More on Speed)