\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  F_r = m g C_r\, \text{sgn}(v),
\end{equation*}
\end{document}