Hi every body this is my first post on these forums. I have a hollow column made from structural steel that has a Young's modulus of 200GN m^-2 and a yield stress of 140MN m^-2 it is 10m long. the larger radius R is 40mm the smaller r is 30mm I am asked to find the load that will cause it to buckle which i have used the formula stress = force/cross sectional area and transposed it to force = stress x cross sectional area so force = (140 x 10^6 ) x (2.199 x 10^-3) = 307.86 x 10^3 N then the next question asks me to do the same but for a column of half the length ... but if i do this the answer will just be the same because the equation for stress does not take the length into account. So i think i am doing something wrong in the first part of the question as well, otherwise the next part seems pointless. I am not brilliant at mathematics so any help hints or advice would be appreciated. thanks in advance.