\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\aligned
  (M+m)\frac{d^2{\color{blue} p}}{dt^2} - m l \cos{\theta} \frac{d^2{\color{blue} \theta}}{dt^2} + m l
  \sin{\theta} \bigl(\frac{d\theta}{dt}\bigr)^2 &=F, \\
  -m l \cos{\theta} \frac{d^2{\color{blue} p}}{dt^2} + (J + m l^2) \frac{d^2\theta}{dt^2} - m g
  l \sin{\theta} &= 0.
\endaligned
\end{equation*}
\end{document}