Consider a harmonically forced system given by$,$ md$2$x$($t$)$dt$2+$ cdx$($t$)$dt $+$ kx$($t$)=$ Fo$($wt$),$ where m$=2$kg$,$ c$=1$ N$-$s$/$m$,$ k$=5$ N$/$m$,$ and Fo$=10$ N$.$ As the input frequency w is varied from $0$ to infinity rad$/$s$,$ the maximum steady$-$state amplitude occurs when the input frequency is $\_\_\_\_$ rad$/.$