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A fan rotating at 90 rpm stops in 21 rotations under constant frictional torque. The time taken is
A21 s
B14 s
C28 s
D42 s
Answer & Solution
Correct answer: C. 28 s
1. Initial angular speed ω₀ = 2π(1.5) = 3π rad/s; final ω = 0.
2. Angle covered θ = 2π(21) = 42π rad.
3. Using α = (ω−ω₀)/t = (ω²−ω₀²)/(2θ): (0−3π)/t = (0−9π²)/(84π).
4. Solving gives t = 28 s.
_Source: Maharashtra Balbharati Std XII Physics, Ch 1 "Rotational Dynamics" §1.2.1_
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