Question

Seven identical circular planar disks, each of mass M and radius R are welded symmetrically as shown. The moment of inertia of the arrangement about the axis normal to the plane and passing through the point P is :

(1) 55/2 MR² (2)73/2 MR²(3)181/2 MR² (4)19/2 MR²
 

Answer 1

Ans.(3)

so,option (3) is the correct option

Comments

The last step wrongly uses the value 3R for distance between the centre of mass and the axis of rotation. The value should be 2R

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From the point O to P radius is (R + R)
And before the point O radius is R
So summation of Radius is (R+R+R) = 3R
So the last step is correct.