\documentclass[12pt]{article}

\begin{document}

$$
   F_{hk} = (k_n \delta \mathbf{n}_{ij} -  
   m_{\mbox{\scriptsize{eff}}} \gamma_n\mathbf{ v}_n) - 
   (k_t \mathbf{ \Delta s}_t +
   m_{\mbox{\scriptsize{eff}}} \gamma_t \mathbf{v}_t)
$$

\end{document}
