A ring of radius r is first rotated with an angular velocity of w0 and then carefully placed on a rough horizontal surface.

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A ring of radius r is first rotated with an angular velocity of w0 and then carefully placed on a rough horizontal surface. The coefficient of friction between the surface and the ring is μ , then the time after which its angular speed becomes 2/3is

(a) $\frac{\omega_0r}{3μg}$

(b) $\frac{\omega_0r}{2μg}$

(c) $\frac{3\omega_0r}{2μg}$

(d) $\frac{\omega_0r}{μg}$

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Answer is (a) $\frac{\omega_0r}{3μg}$