Question

A wheel turning with angular speed of 30rev/s is brought to rest with a constant acceleration. It turns 60 rev before it stops. The time that elapses before it stops is:

• A. 2 s
• B. 4 s
• C. 5 s
• D. 6 s

Answer 1

The correct option is B 4 s

$w=30rev/s,\theta =60rev{w}_f^2=w_1^2-2\alpha \theta \_\_\_\left(1\right)$

$w_f=00^2=900rev/s^2-2\times \alpha \times 60rev\alpha =\frac{900}{120}rev/s^2\alpha =75rev/s^2$

$w_f=w_i-\alpha t(w_f=0,w_i=30rev/s,\alpha =7.5)t=\frac{w_i-w_f}{\alpha }t=\frac{30}{7.5}st=4s$