Question

An electric fan is rotating at a speed of 600 rev/minute. When the power supply is stopped, it stops after 60 revolutions. The time taken to stop is

• A. 12 s
• B. 30 s
• C. 45 s
• D. 60 s

Answer 1

Answer: (A)

12 s

Answer 2

$\omega_{1} = 600rev/\min = 10rev/sec,$ $\omega_{2} = 0\text{and}\theta = 60rev$From the equation $\omega_{2}^{2} = \omega_{1}^{2} - 2\alpha\theta \Rightarrow$ $0 = ( 10 ) ^ { 2 } - 2 \alpha 60$

$\therefore\alpha = \frac{100}{120} = \frac{5}{6}$

Again $\omega_{2} = \omega_{1} - \alpha t \Rightarrow 0 = \omega_{1} - \alpha t$

$t = \frac{\omega_{1}}{\alpha} = \frac{10 \times 6}{5} = 12sec$.