Two disks are mounted on frictionless bearings on horizontal axles. They are bro

Question

Two disks are mounted on frictionless bearings on horizontal axles. They are brought together horizontally so that they stick together and rotate as one unit. The first disk, with rotational inertia 3.0 kg·m2 about its central axis, is set spinning at (+)550 rev/min. The second disk, with rotational inertia 6.0 kg·m2 about its central axis, is set spinning at (+)700 rev/min in the same direction as the first. They then come together and rotate as one unit.

(a) Let the system include both disks. By assuming that they are displaced in the horizontal direction only and the bearings are frictionless, we are really making which of the following assumptions (choose which are correct)

-there are no unbalanced external torques acting in the horizontal direction on the system

-the angular momentum of the system in the horizontal direction is conservedthe mechanical energy of

-the system is conservedthe kinetic energy of

-the system is conserved
(b) What is their angular speed after coupling?

(c) If instead the second disk is set spinning at 700 rev/min in the direction opposite the first disk's rotation (consider the first disk's direction of rotation to be positive and the second disk's direction of rotation to be negative), what is the angular velocity after coupling?

Explanation / Answer

- ANGULAR MOMENTUM IS CONSERVED ( CHOICE B)

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ANGULAR SPEED = (3*550 + 6*700)/9   =650

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NOW ,SPEED   = ( 3*550 -6*700 )/9   = -283.33

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