Question

A horizontal flat platform is rotating with uniform angular velocity around the vertical axis passing through its centre. At some instant a viscous fluid of mass $m$ is dropped at the centre and is allowed to spread out and finally fall. The angular velocity during this period :

• A. decreases continuously
• B. decreases initially and increases again
• C. remains unaltered
• D. increases continuously

Answer 1

Answer: (B)

decreases initially and increases again

The liquid when initially spreads towards the circumference of the platform, its rotational inertia starts increasing, as more and more mass is distributed further from the center. This reduces the angular velocity using the principle of conservation of angular momentum. And when the fluid finally drops from the platform the rotational inertia decreases again thereby increasing the angular velocity again.