The uniform semicircular bar of radius r = 80 mm and mass m = 4.0 kg rotates freely about a horizontal axis through the pivot O. The bar is initially held in position 1 against the action of the torsional spring and then suddenly released. Determine the spring stiffness kT which will give the bar a counterclockwise angular velocity ω = 4.1 rad/s when it reaches position 2, at which the spring is undeformed.