Velocity is a physical vector quantity; both magnitude and direction are needed to define it. The scalar absolute value (magnitude) of velocity is called speed, a quantity that is measured in metres per second (m/s or m⋅s −1) in the SI (metric) system.hus, a constant velocity means motion in a straight line at a constant speed.. For example, a car moving at a constant 20 kilometres per hour in a circular path has a constant speed, but does not have a constant velocity because its direction changes. Hence, the car is considered to be undergoing an acceleration.