Two disks of identical mass but different radii (r and 2r) are spinning on frict

Question

Two disks of identical mass but different radii (r and 2r) are spinning on frictionless bearings at the same angular speed omega_0, but in opposite directions. The two disks are brought slowly together. The resulting frictional force between the surfaces eventually brings them to a common angular velocity. (a) What is the magnitude of that final angular velocity in terms of omega_0? (Use the following as necessary: omega_0.) omega_f = 3/5 omega_0 (b) What is the change in rotational kinetic energy of the system? (Use the following as necessary: K as the initial kinetic energy.) Delta k = Explain.

Explanation / Answer

Let the moment of th einertia of the system be I

Initial rotational kinetic energy of the system K = (1/2)Iwo 2

Final rotational kinetic energy of the system K ' = (1/2) Iwf 2

K ' = (1/2) I[(3/5)wo] 2

= (9/25) (1/2)Iwo 2

Change in rotational kinetic energy dK = K - K '

dK = [(1/2)Iwo 2 ] -[(9/25)(1/2)Iwo 2 ]

= [(1/2)Iwo 2 ] [ 1-(9/25) ]

= [(1/2)Iwo 2 ](16/25)

= (16/25) K

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