Question

A horizontal platform is rotating with uniform angular velcity around the vertical axis passing through its centre. At some instant of time 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. decrease continuously
B. decreases initially and increases again
C. remains unaltered
D. increases continuously

Answer 1

Correct Answer - B
From law of conservation of angular momentum if no external torque is acting upon a body rotating about an axis, then the angular momentum of the body remains constant that is
`J = I omega =` constant
Hence, if `I` decreases , `omega` increases and vice-versa. Hence liquid is dropped, mass increases hence `I` increases `(I = mr^2)`. So, `omega` decreases, but as soon as the liquid starts falling `omega` increases again.