Question

The moment of inertia of a ring about a tangent is 4kgm^2. What is the moment of inertia about an axis passing through the centre of the ring and perpendicular to its plane? Mass of the ring is 2kg & diameter is 2m.

(a) 2kgm^2

(b) 4kgm^2

(c) 8kgm^2

(d) 1kgm^2

I have been asked this question in a job interview.

The origin of the question is Theorems of Perpendicular and Parallel Axes topic in chapter System of Particles and Rotational Motion of Physics – Class 11

Answer 1

The correct choice is (b) 4kgm^2

Easy explanation: Using parallel axis theorem we can find I about centre of mass.

∴ I = 4 – 2*1 = 2kgm^2.

Now using perpendicular axis theorem we get I1 about the desired axis.

∴ I1 = I + I = 4kgm^2.