Four the same spheres connected to the ends of two rods are rotating on a plane (no friction). Asuume the bar as a cross and the whole as a disk. At any point on the rod, the magnitudes of the centrifugal force and centripetal force are equal, as shown by Newton's third law of motion. That is, centrifugal force (and centripetal) is not fictitious to an observer in any frame.
