The two masses (m1 = 5.0 kg and m2 = 3.0 kg) in the Atwood's machine shown in the figure are released from rest, with m1 at a height of 0.91 m above the floor. When m1 hits the ground its speed is 1.0 m/s. Assume that the pulley is a uniform disk with a radius of 12 cm. Determine the pulley's mass.