A boy is seated in a revolving chair revolving at an angular speed of 120 rev.per minute. Two heavy balls form part of the revolving system and the boy can pull the balls closer to himself or may push them apart. If by pulling the balls closer the boy dcreases the Moment of Inertia from 6 kgm2 to 2 kgm2. What will be new angular speed?

You just need to conserve the angular momentum.

Angular momentum = Iω

where, I is the moment of inertia and ω is the angular speed.

So, initially,

angular momentum = Iω = 6 x 120 = 720

Finally,

angular momentum = I'ω' = 2 x t    (where t is the new angular speed)

Since, initial and final momentum is conserved, therefore,

6 x 120 = 2 x t

t = 360

Therefore, new angular speed is 360 revolutions per minute.

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