Question

Seven identical circular planar discs, 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

• A. $\frac{73}{2}{MR}^2$
• B. $\frac{181}{2}{MR}^2$
• C. $\frac{19}{2}{MR}^2$
• D. $\frac{55}{2}{MR}^2$

Answer 1

The correct option is B $\frac{181}{2}{MR}^2$

$I_p=I_0+7M(3R{)}^2$
$=(\frac{{MR}^2}{2}+6(\frac{{MR}^2}{2}+M(2R{)}^2))+7M(3R{)}^2$
$=\frac{181}{2}{MR}^2$