Question

Two disks are rotating about the same axis. Disk A has a moment of inertia of 4.53 kg·m2 and an angular velocity of +4.55 rad/s. Disk B is rotating with an angular velocity of -6.86 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 -2.24 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

Angular momentum is conserved.
Ia*wa + Ib*wb = (Ia + Ib)*wfinal
4.53*4.55 - 6.86*Ib = (4.53+Ib)*2.24

Solve for Ib.

Answer 2

two discs are located on the same axis , the lower disc has inertia 8kg/m2 and it is intialy rotating with angular velocity of 4 rad/sec and the upper one has inertia 4kg/m2 is not rotating the upper disc then drops in to the lower disc and stic togther and reach the same angular velocity. find the common velocity