The steel $($E $=200$ GPa$)$ bar AB shown in Figure $1$ has a hollow circular cross section, with externaland internal radii ro $=18$ mm and r; $=12$ mm$,$ respectively. The compressive Yield strength of the steelgrade used for the bar is S$,=450$ MPa. Determine the maximum downward force F that can be appliedto the bar in B without failure, if:$($a$)$ The length of the bar is L $=3$ m$.($b$)$ The length of the bar is L $=0\mathrm{.}65$ m$.$