The limiting stress of a typical human bone is 0.9 × 10⁸ Nm^–2 while young's modulus is 1.4 × 10¹⁰ Nm^–2. How much energy can be absorbed by two leg bones without breaking if each has a typical length of 50cm and an average cross sectional area of 5cm² ? 
 

Now limiting stress,

$\mathrm{Limit} \mathrm{stress}=0.9 \times {10}^8{\mathrm{Nm}}^{-2}$

$\mathrm{Y}=1.4\times {10}^{10}{\mathrm{Nm}}^{-2}$

$\mathrm{Length} \mathrm{of} \mathrm{both} \mathrm{the} \mathrm{legs},\mathit{ }l = 2 \times 50$

$l= 100 \mathrm{cm} = 1.0 \mathrm{m}$

$\mathrm{A}=5{\mathrm{cm}}^2= 5\times {10}^{-4}{\mathrm{m}}^2$

Stretching force,

$\mathrm{F} = \mathrm{Stress} \times \mathrm{area} = 0.9 \times {10}^8\times 5\times {10}^{-4}=4.5\times {10}^4\mathrm{N}$

$\mathrm{Y}=\frac{\mathrm{Stress}}{{\displaystyle \frac{∆l}{l}}}\Rightarrow ∆l=\frac{\mathrm{stress}\times l}{\mathrm{Y}}=\frac{0.9\times {10}^8\times 1.0}{1.4\times {10}^{10}}=6.43\times {10}^{-3}\mathrm{m}$

Elastic potential energy 

$\mathrm{U}=\frac{1}{2}\times \mathrm{F}\times ∆l=\frac{1}{2}\times 4.5\times {10}^4\times 6.43\times {10}^{-3}=144.7\mathrm{J}$

Hence the correct answer is 145J.