Q. Two rods each of mass $m$ and length $l$ are joined at the centre to form a cross. The moment of inertia of this cross about an axis passing through the common centre of the rods and perpendicular to the plane formed by them, is
A) ${ml}^{2}/12$
B) ${ml}^{2}/6$
C) ${ml}^{2}/3$
D) ${ml}^{2}/2$
Explanation
(b) When $l$ is length of rod and its mass, then
the moment of inertia of its axis passing
through its centre of gravity and perpendicular
to its length is given by
[image:7_image110.emf]
$I = \frac{ml^{2}}{12}$
For two rods as shown,
$I = \frac{ml^{2}}{12} + \frac{ml^{2}}{12} = \frac{ml^{2}}{6}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.