Description: Instantaneous speed is a measure of speed at a single moment in time. Average speed is a measurement over a longer distance and time. Velocity is a measurement of speed and direction.
Key Terms: Instantaneous speed, average speed, total distance, meters per second, velocity, vector, magnitude, direction

Constant speed means steady speed. Something with constant speed doesn’t speed up or slow down. Constant velocity, on the other hand, means both constant speed and constant direction. Constant direction is a straight line—the object’s path doesn’t curve. So constant velocity means motion in a straight line at a constant speed.

If either the speed or the direction changes (or if both change), then the velocity changes. A car on a curved track, for example, may have a constant speed, but, because its direction is changing, its velocity is not constant. We’ll see in the next section that it is accelerating.