Le produit vectoriel donne sans piège :

\mathbf{a} \wedge {\sigma\hspace{-0.58em}\sigma\hspace{-0.59em}\sigma} = -{ {\mathcal{G}} M\over r^{2}}\ \mathbf{u} _{\mathrm{r}} \wedge mr^{2}{\dot\theta}\ \mathbf{u} _{\mathrm{z}} = + {\mathcal{G}} M m {\dot\theta}\ \mathbf{u}_\theta