Question

Two disks are rotating about the same axis. Disk A has a moment of inertia of 3.3 kg · m² and an angular velocity of +7.4 rad/s. Disk B is rotating with an angular velocity of -9.3 rad/s. The two disks are then linked together without the aid of any external torques, so that they rotate as a single unit with an angular velocity of -2.5 rad/s. The axis of rotation for this unit is the same as that for the separate disks. What is the moment of inertia of disk B?

Answer 1

IA =3.3 kg.m^2 ,wA = 7.4 rad/s, wB = -9.3 rad/s, w = -2.5 rad/s

From conservation of angular momentum

IA*wA +IB*wB = (IA +IB)w

(3.3*7.4) - (IB*9.3) = -(3.3+IB)(2.5)

IB =4.8 kg.m^2