The radius of gyration of a uniform thin rod of length $L$ about an axis passing normally through its centre of mass is:

Two masses $m_1$ and $m_2$ are placed at a distance $r$ from each other. Find the moment of inertia of the system about an axis passing through the center of mass and perpendicular to the line joining the masses.

Two rings of radius $R$ and $nR$ made of the same material have a ratio of moment of inertia about an axis passing through their centers and perpendicular to their planes as $1:8$. The value of $n$ is (mass per unit length $= \lambda$).

Two discs $A$ and $B$ of same material and thickness have radii $R$ and $3R$ respectively. Their moments of inertia about their axis will be in the ratio

$A$ thin wire of length $l$ and mass $M$ is bent in the form of a semi-circle. What is its moment of inertia about an axis passing through the ends of the wire?

What is the $SI$ unit of the radius of gyration?