Question

A fly wheel rotating at 420 r.p.m. slows down at a constant rate of $2rad/s^2$. What time is required to stop the fly wheel ? (in s)

• A. $96$
  
• B. $22$
  
• C. $32$
  
• D. $15$
  

Answer 1

The correct option is B $22$

Initial angular velocity ${\omega }_0=420rpm=420\times \frac{2\pi }{60sec}=14\pi rad/s$

Final angular velocity ${\omega }_t=0rad/s$

Given : $\alpha =-2rad/s^2$

As we know, ${\omega }_t={\omega }_0+\alpha t$

$\Rightarrow t=\frac{{\omega }_t-{\omega }_0}{\alpha }=\frac{-14\pi }{-2}=22sec$