B 17.54 Two disks of the same thickness and same material are attached to a shaft as shown. The 4 kg disk A has a radius rA = 60 mm, and disk B has A a radius rg = 90 mm. Knowing that a couple M of magnitude 2 N . m is ap- M plied to disk A when the system is at rest, determine the time required for the angular velocity of the system to reach 960 r/min. Fig. P17.54 and P17.55