The time rate of change of position of a body without regard to direction. It is the numerical magnitude of a velocity and hence is a scalar quantity. Linear speed is commonly measured in such units as meters per second, miles per hour, or feet per second. It is the most frequently mentioned attribute of motion.
Average linear speed is the ratio of the length of the path Δs traversed by a body to the elapsed time Δt during which the body moved through that path, as in Eq. (1),
where s0 and t0 are the initial position and time, respectively, sf and tf are the final position and time, and Δ stands for “the change in.”
Instantaneous speed, defined by Eq. (2),
is the limiting value of the foregoing ratio as the elapsed time approaches zero. See also: Velocity