Question

A motor of an engine is rotating about its axis with an angular velocity of $100$ rev/min . It comes to rest in $15$ s, after being switched off. Assuming constant angular deceleration, calculate the number of revolutions made by it before coming to rest.

• A. $10.5$
• B. $12.5$
• C. $15.0$
• D. $20.5$

Answer 1

The correct option is B $12.5$

Deceleration can be found by using the equation of motion

$\omega ={\omega }_0+\alpha t$

$⟹\alpha =-\frac{{\omega }_0}{t}$$=-\frac{100}{15\times 60}rev/s^2$$=-\frac{1}{9}rev/s^2$

Now using the equation of motion:

${\omega }^2-{\omega }_0^2=2\alpha \theta$

$⟹\theta =\frac{{\omega }_0^2}{2\alpha }$$=12.5$