Question

A disc of mass m and radius R has a concentric hole of radius r. Its moment of inertia about an axis through its centre and perpendicular to its plane is

(a) 1/2 m(R - r)²

(b) 1/2 m(R² - r²)

(c) 1/2 m(R + r)²

(d) 1/2 m(R² + r²)

Answer 1

Correct Answer is: (d) 1/2 m(R² + r²)

Mass of the elemental ring = (2πx dx) ( m / πR² - πr²) = dm.

Moment of inertia of the ring = (dm) x².