The reel shown in figure has radius R and moment of inertia I. One end of the block of mass m is connected to a spring of force constant k, and the other end is fastened to a cord wrapped around the reel. The reel axle and the incline are frictionless. The reel is wound counterclockwise so that the spring stretches a distance d from its un-stretched position and the reel is then released from rest. The angular speed of the reel when the spring is again un-stretched is
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a
2mgd sin θ +kd2I+ mR2
b
mgd sin θ + kd2I + mR2
c
mgd sin θ-kd2I+mR2
d
2mgd sin θ-kd2I+mR2
answer is A.
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Detailed Solution
The block and end of the spring are pulled a distance d up the incline and then released. The angular speed of the reel and the speed of the block are related by v = ωR. The block-reel-Earth system is isolated, so∆K +∆U = 0(12mv2-0)+(12Iω2-0)+(0-mgd sin θ)+(0-12kd2) = 012ω2(I+mR2) = mgd sin θ + 12kd2⇒ω = 2mgd sin θ +kd2I + mR2