A steel rod 2.5 m long, A = 1200 mm^{2}, is secured between two walls with zero load at 20 °C. Find the stress (MPa) in the rod when the temperature falls to −20 °C, (a) if the walls are rigid and (b) if the walls move towards each other by a total of 0.500 mm as the temperature drops. Take α = 11.7 µm/(m·°C) and E = 200 GPa.