$$ y=\frac{\text { stress }}{\text { strain }}=2.0 \times 10^{10} $$ Energy density $=\frac{1}{2}$ stress × strain $$ \begin{aligned} & =\frac{1}{2}(\text { strain })^2 y=\frac{1}{2}\left(5 \times 10^{-4}\right)^2 \times 20 \times 10^{10} \\ & =25 \times 10^2 \times 10=25 \frac{\mathrm{~kJ}}{\mathrm{~m}^3} \end{aligned} $$ Ans. 25