From a circular disc of radius R and mass 9M a small disc of radius R/...
The moment of inertia of remaining disk = Moment of inertia of whole disc - Moment of inertia of removed small disc
moment of inertia of any disk about its own central axis = 1/2 mr^2
Moment of inertia of total disk of mass 9M and radius R = (1/2) 9M R^2
Iwhole = (1/2) 9M R^2
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Let us calculate the mass of the removed disk and the location of its centre of mass
mass density = 9M / area of disk = 9M/ piR^2
mass of small disk of radius R/3 = area of small disk x mass density = pi(R/3)^2 x 9M/ piR^2
We get the mass of the removed small disk = M
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Its centre of mass is located at R - R/3 = 2R/3 from the main axis
its moment of inertia can be calculated using parallel axis theorem
moment of inertia of removed disk = moment of inertia of small disk about its own axis + moment of inerita of its centre of mass about the main axis
Iremoved = (1/2) M (R/3)^2 + M (2R/3)^2
= MR2 /18 + 4MR^2 /9
Iremoved = MR^2 /2
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hence the moment of inertia of remaining disk = Iwhole - Iremoved
= (1/2) 9M R^2 - MR^2 /2
= 4MR^2