\documentclass[12pt]{article}

\begin{document}

$$

   F_{hz} = \sqrt{\delta} \sqrt{\frac{R_i R_j}{R_i + R_j}} F_{hk} = 

     \sqrt{\delta} \sqrt{\frac{R_i R_j}{R_i + R_j}} 

     \Big[ (k_n \delta \mathbf{n}_{ij} -  

       m_{\mbox{\scriptsize{eff}}} \: \gamma_n \mathbf{ v}_n) -

       (k_t \delta\mathbf{ \Delta s}_t +

       m_{\mbox{\scriptsize{eff}}} \: \gamma_t \mathbf{v}_t) \Big]

$$

\end{document}

\documentclass[12pt]{article}

\begin{document}

$$

   F_{hk} = (k_n \delta \mathbf{n}_{ij} -  

   m_{\mbox{\scriptsize{eff}}} \gamma_n\mathbf{ v}_n) - 

   (k_t \delta\mathbf{ \Delta s}_t +

   m_{\mbox{\scriptsize{eff}}} \gamma_t \mathbf{v}_t)

$$

\end{document}