Question

The speed of a motor increases from 1200 rpm to 1800 rpm in 20s. How many revolutions does it make in this period of time?

• A. 100
• B. 200
• C. 500
• D. 800

Answer 1

The correct option is C 500

Given,

${\omega }_0=1200rpm=20rps$

$\omega =1800rpm=30rps$

$t=20sec$

From the rotational mechanics

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

$\alpha =\frac{\omega -{\omega }_0}{t}$

$\alpha =\frac{30-20}{20}=0.5rp{s}^2$

The number of rotation,

$\theta ={\omega }_{0}t+\frac{1}{2}\alpha t^2$

$\theta =20\times 20+\frac{1}{2}\times 0.5\times 20\times 20$

$\theta =400+100=500rotation$

The correct option is C.