Question

Two thin discs, each of mass M and radius r meter, are attached as shown in the fig. to form a rigid body. The rotational inertia of this body about an axis perpendicular to the plane of disc B passing through its centre is

Answer 1

The moment of inertia of the disc about the centroidal axis is Mr²​/2.

The moment of inertia of the disc at A about the axis passing through B is given using parallel axis theorem as Mr² ​+ M(2R)²

Thus the total moment of inertia of the whole system is given as Mr²​/2 + Mr² /2 ​+ M(2R)² = 5Mr².