Question

A horizontal disc rotating freely about a vertical axis makes 90 revolutions per minute reduces to 60,then find the moment of inertia of the disc?
1. mr2
2. 3/2mr2
3. 2mr2
4. 3mr2

(the last ‘2’in each option should be considered as square, please)

More information is required on the torque and how long it takes to slow down. If it is "rotating freely" then why is it slowing down at all?

Your question is incomplete. What effort have you made to solve it yourself?

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