A thin circular ring of mass M and radius R is rotating in a horizontal plane about an axis vertical to its plane with a constant angular velocity ?. If two objects each of mass m be attached gently to the opposite ends of a diameter of the ring, the ring will then rotate with an angular velocity:

Options

(a) ?M / M + 2m
(b) ? (M + 2m) / M
(c) ?M / M + m
(d) ? (M – 2m) / M + 2m

Correct Answer:

?M / M + 2m

Explanation:

In abstance of external torque, L = I? = constant
I₁?￢ﾂﾁ = I₂?￢ﾂﾂ,I₁ = MR²,I₂ = MR² + 2mR²
(Moment of inertia of a thin ciruclar ring about axis vertical to its plane = MR²)
?￢ﾂﾂ = I₁ / I₂ ⇒ ? = ?M / M + 2m