A $3 m$ long, solid circular steel column is securely fixed at its base into a concrete footing and completely free at its top (acting like a flagpole). Its diameter is $50 mm$ and its modulus of elasticity is $E = 200 GPa$. Determine its critical buckling load.

A hollow square steel tube is used as a column. Its outer dimension is $100 mm$ and its wall thickness is $5 mm$. The column is $4 m$ long, fixed at its base and pinned at its top. Given $E = 200 GPa$, find the critical buckling load.