Q.

A circular hole of radius R/4 is made in a thin uniform disc having mass M and radius R, as shown in figure. The moment of inertia of the remaining portion of the disc about an axis passing through the point O and perpendicular to the plane of the disc is :

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a

219MR2256  

b

197MR2256 

c

19MR2512 

d

237MR2512

answer is D.

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Detailed Solution

Let the disc's mass per unit area be σ.
now, removed mass's moment of inertia about the origin is

Using the parallel axis theorem,

M2=(σA')R422+(σA')3R42

M2=σA'R232+9R216

A'=πR42=πR216

M2=σπR4161932

The moment of inertia of the complete disc,

M1=σA×R22

Effect moment of inertia,

=M1-M2

Mo=σ2πR4-σπR419512=σπR2R2237512

σπR2=M

Mo=237MR2512

Hence the correct answer is 237MR2512.

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