Question

The motor of an angle is rotating about its axis with an angular velocity of 100 rev/m. It comes to rest in 15 s, after being switched off. Assuming constant angular deceleration. What are the numbers of revolutions made by it before coming to rest?

• A. 12.5
• B. 40
• C. 32.6
• D. 15.6

Answer 1

Answer: (A)

12.5

Answer 2

From equation of motion $0= \omega_0 - \alpha t$ $\alpha = \frac{\omega_0}{t}$ = $\frac{(100 \times 2\pi)/60}{15}$ = $0.6 \,rad/s^2$ Now, angle rotated before coming to rest $\theta = \frac{\omega^2_0}{2\,\alpha}$ or $\theta= \frac{\left(\frac{100 \times 2\pi}{60}\right)^2}{2 \times 0.7}=78.25 \,rad$ Number of rotations $n = \frac{\theta}{2\pi} = 12.5$