documentclass{article}
usepackage{subcaption}
usepackage[demo]{graphicx} % remove 'demo' option for your real document
egin{document}
egin{figure}
centering
egin{subfigure}[b]{0.55 extwidth}
includegraphics[width=1linewidth]{Nvariousg1}
caption{}
label{fig:Ng1}
end{subfigure}
egin{subfigure}[b]{0.55 extwidth}
includegraphics[width=1linewidth]{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}