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

Options

A. `2M r^(2)`
B. `3Mr^(2)`
C. `4Mr^(2)`
D. `5Mr^(2)`

Answer

D

Explanation

`I_(B) = 1/2 mr^(2),I_(A)` about the given axis (by parallel axis theorem) `=1/2 mr^(2) + m(2r)^(2) = 9/2 mr^(2)`
`rArr I=1/2mr^(2)+ 9/2 mr^(2) = 5mr^(2)`