Q$2(30$ pts$)$: A steel bar which has a circular cross$-$section and radius of $50mm$ is subjected to an axial force of $400kN.$ The initial length of the bar is $1\mathrm{.}6$ meters. If the radius of the bar decreases $)=-4\mathrm{.}58m=({4\mathrm{.}5810}^{-6}(m).$

Calculate longitudinal elongation on the rod.

Shear G $($Shear Modulus$)$ and Poisson's Ratio $($v$).$