$4$ For the falliswing, bars, determine $($a$)$ due critical load for due muel bar, b$)$ the dimension d for which te aluminu bar will have the same critical load. $($s$)$ Express the weighrof tre alluminum bar as a persent of the weight of the stsel fer? $($Sor Sicel E$-200$ GPa andp$-7360$ Kgin and for Aluminum E$-70$ GPa and p$-2300$ Kg$/$m$?)$