Is it meaningful to measure the speed of an object in a rotating frame, and if so how do you do it in the following case? Consider a rim of circumference 100 rotating at 0.8c, both measurements with respect to the inertial lab frame. An object is at a point on the rim with a standard clock affixed there (the origin). The object begins to move around the rim in a counter-rotating direction at 0.8c relative to the rim (as measured in the lab frame). The result is that the object is at rest in the lab frame (like a person running on a treadmill). After lab time 125 the rim will complete one rotation in the lab, and the clock and object will be together again at the origin on the rim. Is it possible to transform this to determine the object's speed in the rim frame? In the rim frame, the rim and the clock are at rest. The object is in motion, moving around the rim. The elapsed time on the affixed clock when the object completes one circuit around the rim and returns to the origin will be 75 because of time dilation (gamma = 1.67). I can think of at least three possibilities. Unlike a Born rigidly accelerating rod, standard clocks affixed to all points on the rim will run at the same rate, so it seems reasonable to use the clock at the origin to measure the total rim time for the object's circuit. 1. If the rim's own circumference equals its ground circumference times gamma, then the object will have traveled rim distance 167 in rim time 75 (greater than c). 2. If the rim's own circumference equals its ground circumference, the object will have traveled rim distance 100 in rim time 75 (greater than c). 3. If the rim's own circumference equals its ground circumference divided by gamma, then the object will have traveled rim distance 60 in rim time 75 (0.8c). Thanks for any help.