Imagine that a car begins traveling along a road after starting from a specific signpost. To know the exact position of the car after it has traveled a given distance, you need to know not only the miles it traveled but also its heading.

The displacement, defined as the change in position of the object, is a vector with the magnitude as a distance, such as 10 miles, and a direction, such as east. Velocity is a vector expression with a magnitude equal to the speed traveled and with an indicated direction of motion. For motion defined on a number line, a positive or negative sign specifies the direction.

Average velocity is mathematically defined as

average velocity = total displacement/time elapsed

Note that displacement (distance from starting position) is not the same as distance traveled. If a car travels one mile east and then returns one mile west, to the same position, the total displacement is zero and so is the average velocity over this time period. Displacement is measured in units of length, such as meters or kilometers, and velocity is measured in units of length per time, such as meters/second (meters per second).