atomic molecule allways has six degrees of freedom, situation which has not been suffi- ciently explained in literature, in spite of its widespread use in the calculation of the in- ternal energies and heat capacities of ideal polyatomic gases. We thank Trinidad Cruz-S´ anchez for his valuable contribution to the fulfillment of this work. [1] G. R. Fowles, Analytical Mechanics , (Holt, Rinehart and Winston, New York, 1962). [2] F. Scheck, Mechanics , (Springer - Verlag, Berlin, 1990). [3] H. Goldstein, C. Poole, and J. Safko, Classi- cal Mechanics , (Addison Wesley, San Fran- cisco, 2003). [4] N. C. Barford, Mechanics , (Editorial Re- vert´ e, Barcelona, 1976). [5] A. Einstein, The Meaning of Relativity , (Princeton University Press, Princeton, 1953). [6] R. Resnick, D. Halliday, and K. S. Krane, F´ısica, Volumen 1 , (CECSA, M´ exico, 1997). [7] G. W. Castellan, Fisicoqu´ımica , (Addison Wesley Longman, 1986). [8] F. Mandl, F´ısica Estad´ıstica , (Editorial Limusa, M´ exico, 1976). [9] T. L. Hill, An Introduction to Statisti- cal Thermodynamics , (Dover Publications, New York, 1976). [10] K. Huang, Statistical Mechanics , (John Wi- ley & Sons, New York, 1963). Figure 1. At instant t = 0 ( a ) the sys- tem is in its initial posistion. The line that conects the centers of mass of two arbitrary particles forms an angle ϕ ( t = 0) with the direction of the vector v orthogonal to the reference plane P . At a future instant t = t ´ ( b ) the system has rotated respect to its orig- inal position. The plane P has moved, but a copy P ´remains in the original position of P , so now v is perpendicular to P ´. And the line that joins the centers of mass of the particles we had considered forms an angle ϕ ( t ´) with the direction of v . Figure Captions 8