Speed is the rate of change of position of a moving body or its rate of motion; velocity is the speed in a definite direction. If the motion is uniform, the speed is measured by the distance the body goes in a unit of time. If the motion is variable, the speed at any instant is the distance it would move during the next unit of time if it should continue to move at the same rate.
If the speed of a body is greater for each unit of time than it was for the preceding, the motion is said to be accelerated. If the acceleration, or increase of speed, is the same for each unit of time, the motion is uniformly accelerated.
Motion is retarded, or negatively accelerated, when the speed is decreasing instead of increasing, and if the retardation is uniform, the motion is uniformly retarded.
Average or mean speed is the speed with which a body would need to move uniformly to pass over a certain space in a given time, though the actual speeds may be made up of a great many rates.
If only motion in a definite direction is considered, the above statements about speed will also hold true of velocity.
