\partial {\bf p}_{{\rm F}/{\rm O}}}{\partial \psi} = - a \sin \psi {\bf i} + a \cos \psi {\bf j}
%     \end{eqnarray}
% </p>
% <p>and</p>
% <p>
%     \begin{eqnarray}
%     \nonumber \frac{\partial {\bf p}_{{\rm R}/{\rm O}}}{\partial q_1} &=& \frac{\partial {\bf p}_{{\rm R}/{\rm O}}}{\partial x} = {\bf i} \\
%     \nonumber \frac{\partial {\bf p}_{{\rm R}/{\rm O}}}{\partial q_2} &=& \frac{\partial {\bf p}_{{\rm R}/{\rm O}}}{\partial y} = {\bf j} \\
%     \nonumber \frac{\partial {\bf p}_{{\rm R}/{\rm O}}}{\