Since it's about the moment of inertia, this is something a rotating body has But first a mathematical way to define a rotating point on a circle. It traces out an arc, s, which is equal to the angle times the radius. Please Register or Log in to view the hidden image! But the angle is only defined if there's are two points on the circle; assume the horizontal radius line is fixed and vary the angle. The point of interest is at the end of r in the diagram, hence r is a position vector and the point it identifies has a tangential velocity. Of course when you physically rotate a particle with mass in a circular path about the centre, there are no little arrows that pop up out of nowhere, what you see is motion. The tangential velocity, the momentum and so on are abstractions, but well-defined ones.