$($a$)$ What do you mean by thermal stress and thermal strain? Find the relevam expressions for thermal stress and strain.

$2+4$

$($b$)$ A steel wire $10$ mm in diameter is used for hoisting purposes in building construction. If $150$ m of the wire is hanging vertically and a load of $800$ N is being lifted at the lower end of the wire, determine the total elongation of the wire. The weight density of steel is $7\mathrm{.}8104\frac{N}{m^3}$ and $E=200G\frac{N}{m^2}.$

$($c$)$ Deduce an expression for the strain produced in a thin cylindrical shell subjected to an internal pressure $p_i.$