Question

Two thin uniform rings made of the same material and of radii $R$ and $4R$ are joined as shown. The mass of the smaller ring is $m$ . Find the moment of inertia of the system about an axis passing through the centre of mass of bigger ring and perpendicular to the plane.

• A. $85mR^{2}$
• B. $90mR^{2}$
• C. $70mR^{2}$
• D. $35mR^{2}$

Answer 1

Answer: (B)

$I=I_{l arger}+I_{s m a l l e r}$
$I=4m\left(\right. 4 R \left.\right)^{2}+\left[m R^{2} + m \left(\right. 5 R \left.\right)^{2}\right]=90mR^{2}$