Question

A boy is seated in a revolving chair revolving at an angular speed of 120 rpm. By some arrangement , the boy decreases the moment of inertia of the system from 6 kg $m2$ to 2 kg $m2$. What will be the new angular speed?

Answer 1

We 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