\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}