What is the section modulus of a square section with side length 'a' about its central axis parallel to a side?

Mohr's circle reduces to a point when an element is subjected to:

A steel column pinned at both ends has an Euler buckling load of 250 kN. If the column is restrained against lateral movement at its mid-height, its new buckling load will be:

What is the angle between the principal strain axis and the maximum shear strain axis in 2D strain analysis?