\documentclass[12pt,preprint]{aastex}
\begin{document}
\begin{equation}
{\bf F = \frac{G M_1 M_2}{D^2} }
\end{equation}
\end{document}
%\input /home/ssm/tex/nutsformat
%$$F = {{G M_1 M_2}\over{D^2}}$$
%\bye


