Solution:
Applying the law of conservation of angular momentum, we have
A thin uniform circular disc of mass M and radius R is rotating in a horizontal plane about an axis passing through its centre and perpendicular to its plane with angular velocity : Another disc of same dimensions
but of mass M/4 is placed gently on the first disc co-axially. The angular velocity 'of the system is
Applying the law of conservation of angular momentum, we have