Rotational speed
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Rotational speed (sometimes called speed of revolution) indicates, for example, how fast a motor is running. Rotational speed is similar to angular speed but not the same. Rotational speed tells how many complete rotations there are per time unit, and it is measured in revolutions per second (1/s or Hertz) in the SI System. The units revolutions per minute (RPM or 1/min) are more common in everyday life. Angular speed, however, tells the change in angle per time unit, measured, for example in radians per second.
For example, a stepper motor might turn exactly one complete revolution each second. Its angular speed is 360 degrees per second (360°/s), or 2π radians per second (2π rad/s), while the rotational speed is 60 RPM.
Rotational speed is not to be confused with tangential speed, despite some relation between the two concepts. Imagine a rotating merry-go-round. No matter how close or far you stand from the axis of rotation, your rotational speed will remain constant. However, your tangential speed does not remain constant. If you stand two meters from the axis of rotation, your tangential speed will be double the amount if you were standing only one meter from the axis of rotation.
