Using the two theoretical predictions of fracture strength, (A) σt = E/π (Equation ( 14.4)) and (B) σt = √(γE/ao) (Equation (14.8)), calculate the theoretical fracture strength of (a) iron and (b) MgO. For iron take ao as its atomic diameter (0.124 nm), and for MgO as the average of the ionic diameters of Mg+2 and O-2 (0.105 nm). The surface energies of iron and MgO are about 2.0 and 1.2 J/m2, respectively. The Young's moduli of iron and MgO are about 270 GPA and 300 GPa, respectively. How do these answers compare with each other?How do these answers compare with the actual fracture strengths of each?