\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
   \begin{align*}
     \dot{\mathbf r}_i &= \mathbf v_i,  \\
     \dot{\mathbf v}_i &= \frac{\mathbf f_i}{m_i} + \frac{\mathbf g_i}{m_i}.
   \end{align*}
\end{document}
