$($b$)$ A bar $4$ m long and $100200m{m}^2$ in cross$-$section is subjected to a pull of $50$ kN $.$

If the Young's modulus of the material of the rod is $2000k\frac{N}{m}{m}^2,$ find $($i$)$ stress developed in the bar, $($ii$)$ strain produced, $($iii$)$ elongation of the rod and $($iv$)$ work done.