Abstract: A covariant four-tensor rotation equation-for bi-dimensional composite body-, by generalising cross product to four-vectors, is obtained. From it, a relativistic angular impulse-angular momentum variation equation (Poinsot-Euler rotation equation) and its pseudo-work-rotational kinetic energy variation equation (Poinsot-Euler pseudo-work equation), are obtained for a body spinning-with constant angular momentum direction-by external torques. Two rotational processes are analysed by using this four-tensor formalism-completed by a four-vector fundamental equation (Newton's second law and thermodynamics first law)-: a rotating body subjected to conservative and friction forces torque-a mechanical energy dissipation process-, and a device spinning by torque produced by chemical reactions-a mechanical energy production process.

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