\documentstyle[12pt]{article} 
\pagestyle{empty}
\begin{document}
\large
\begin{eqnarray} 
\sf 
\vec{r}_{n+1} & = & \vec{r}_n + \frac{\vec{p}_nh}{m} +
\frac{\vec{F}_nh^2}{2m} \nonumber \\
\vec{F}_{n+1} & = & \vec{F}(\vec{r}_{n+1}) \nonumber\\
\vec{p}_{n+1} & = & \vec{p}_n + \frac{\vec{F}_nh}{2} + \frac{\vec{F}_{n+1}h}{m} \nonumber
\end{eqnarray}
\end{document}
\end{document}
