Rotational speed (or rotational frequency) involves the number of revolutions per unit of time. All parts of a rigid merry-go-round or turntable turn about the axis of rotation in the same amount of time. Thus, all parts share the same rate of rotation, or the same number of rotations or revolutions per unit of time. It is common to express rotational rates in revolutions per minute (RPM). When a direction is assigned to rotational speed, it is known as rotational velocity, a vector whose magnitude is the rotational speed. (Angular speed and angular velocity are related to the rotational speed and velocity by a factor of 2 π, the number of radians turned in a full rotation.)