Two disks are mounted on frictionless bearings on the same axle and can b brough

Question

Two disks are mounted on frictionless bearings on the same axle and can b brought together so that they can couple and rotate as one unit. Assume conservation of angular momentum holds. You need to convert rev/min into other units.

(a) The first disk, with rotational inertia 3.3 kg*m^2 about its central axis , is set spinning at +450 rev/min. The second disk, with rotational inertia 6.6 kg*m^2 about its central axis, is set spinnign at +900 rev/min. They then couple together. What is thier angular speed after coupling?

(b)If instead the second disk were spinning at -900 rev/min before coupling, what is the angular speed after coupling?

Explanation / Answer

(a). By Angular momentum balance: I1?1 + I2?2 = (I1+I2) ?

=> 3.3x450 + 6.6x900 = 9.9x?
7425/9.9 = ?
?= 750 rev/min

(b). 3.3x450 - 6.6x900 = 9.9x?
? = - 450 rev/min

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