Asimply supported 2024 aluminum beam is 3 ft long and loaded by 2 equal downward concentrated forces at the 1/3 points. the cross section is rectangular with a height of 4 inches and thickness of .5 inches. at the center of the beam a crack extends vertically 0.2" upwards from the bottom (all the way across the thickness). if the fracture toughness for this material is 20.9 ksi*in^(0.5), determine the magnitude of the applied forces necessary to cause the crack to extend