Physics · SYSTEMS OF PARTICLES AND ROTATIONAL MOTION

A child stands at the centre of a turntable with outstretched arms rotating with an initial rotational kinetic energy \( K_0 \). When the child folds their arms, the moment of inertia of the system reduces to \( \frac{2}{5} \) of its initial value. Assuming no external torque acts, the ratio of the new rotational kinetic energy to the initial rotational kinetic energy (\( K_f / K_0 \)) is:

1. A
   \( 2/5 \)
2. B
   \( 5/2 \)
3. C
   \( 4/25 \)
4. D
   \( 25/4 \)