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Q.  A wire frame AOPQB, lying in the horizontal plane, is free to rotate about a vertical axis passing through center C of the same circle and perpendicular to plane of AOPQB. The mass M of the frame is uniformly distributed over its whole length. The moment of inertia of the frame about this axis, is
(OA = QB = r and CP = r the radius of semicircular part) :-

A   M r 2   14   +   3 π 3 π   +   6                                                                             B   M r 2   14   +   3 π π   +   2 C   M r 2 2   π 2   +   π                                                                                 D   1 2 M r 2                                          

Dear student, 
                     here total mass is M   and total length of the wire =2r+πrhence linear mass density(λ)=M2r+πrso moment of inertia of part AOIAO==r2rx2 ·dm=r2rλx2 ·dx=λr33r2r=λ8r33-r33=7λr33=7Mr23(π+2)similarly for QB partIQB= 7λr33=7Mr23(π+2)and moment of inertia for semicircular part =mr2 =λπr3=M2r+πrr3=πM2+πr2so total moment of inertia of the system=IAO+IQB+IOPQ=7Mr23(π+2)+7Mr23(π+2)+πM2+πr2=M(14+3π)(2+π)r2

Regard

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