The moment of inertia of a planar disc about a diameter is 8kgm^2. What is the moment of inertia about an axis passing through its centre and perpendicular to the plane of disc?

(a) 8kgm^2

(b) 16kgm^2

(c) 4kgm^2

(d) 2√2kgm^2

This question was posed to me by my college professor while I was bunking the class.

My doubt stems from Theorems of Perpendicular and Parallel Axes topic in section System of Particles and Rotational Motion of Physics – Class 11

Select the correct answer from above options

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Correct option is (b) 16kgm^2

Easy explanation: The moment of inertia about any diametrical axis will be the same. So, we can consider two perpendicular diameters and use the perpendicular axis theorem to get the moment of inertia about the axis which is perpendicular to the plane.

Thus, moment of inertia = 8+8

= 16kgm^2.