We introduce here the concept of relative space, an extended 3-space which is recognized as the only space having an operational meaning in the study of the space geometry of a rotating disc. Accordingly, we illustrate how space measurements are performed in the relative space, and we show that an old-aged puzzling problem, Ehrenfest's paradox, is explained in this purely relativistic context. Furthermore, we illustrate the kinematical origin of the tangential dilation which is responsible for the solution of Ehrenfest's paradox.
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