This force also is the force due to tension seen by the rotating observers. The rotating observers see the spheres in circular motion with angular rate ω S = ω I − ω R (S = spheres). That is, if the frame rotates more slowly than the spheres, ω S > 0 and the spheres advance counterclockwise around a circle, while for a more rapidly moving frame, ω S < 0, and the spheres appear to retreat clockwise around a circle. In either case, the rotating observers see circular motion and require a net inward centripetal force: