two plots vertically in one figure latex
\documentclass{article}
\usepackage{subcaption}
\usepackage[demo]{graphicx} % remove 'demo' option for your real document
\begin{document}
\begin{figure}
\centering
\begin{subfigure}[b]{0.55\textwidth}
\includegraphics[width=1\linewidth]{Nvariousg1}
\caption{}
\label{fig:Ng1}
\end{subfigure}
\begin{subfigure}[b]{0.55\textwidth}
\includegraphics[width=1\linewidth]{Nvariousg2}
\caption{}
\label{fig:Ng2}
\end{subfigure}
\caption[Two numerical solutions]{(a) Numerical solutions for the small-time system
with a constant-curvature body shape showing the scaled leading-order veritcal
reaction force $N_0$ versus the scaled body mass $M$ for various values of $g$.
Again, $I=M$ for definiteness and $A=0.7$. (b) As for (a) but over a wider range of
values of $M,I$.}
\end{figure}
\end{document}