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When a ceiling fan is switched on, it makes 10 rotations in the first 3 seconds. How many rotations will it make in the next 3 seconds? (Assume uniform angular acceleration.) (a) 10 (b) 20 (c) 30 (d) 40

Answer»

Correct Answer: (c) 30

θ = 2πn = ωit + 1/2 at2

or 2π x 10 = 1/2 a(3)2 or a = 40π / 9.

Let it make N rotations in the first 6 seconds.

2πN = 1/2 a (6)2  or N = 1/2π . 36/2 .40π/9 = 40

∴ the required number of rotations = 40 - 10 = 30.



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