Question

The diameter of a flywheel is 1 m. It has a mass of 20 kg. It is rotating about its axis with a speed of 120 rotations in one minute. Its angular momentum in $kg{m}^2{s}^{-1}$ is

• A. 13.4
• B. 31.4
• C. 41.4
• D. 43.4

Answer 1

Answer: (B)

Here,
Diameter of a flywheel, $D=1m$
Radius of the flywheel, $R=\frac{D}{2}=\frac{1}{2}m$
Mass of the flywheel, $M=20kg$
Angular speed of rotation,
$\omega =120rpm=\frac{2\pi \times 120}{60}rad{s}^{-1}=4\pi rad{s}^{-1}$
Moment of inertia of the flywheel about its axis,
$I=\frac{1}{2}M{R}^2=\frac{1}{2}\left(20kg\right){\left(\frac{1}{2}m\right)}^2$
$=2.5kg{m}^2$
Angular momentum, $L=I\omega$
$=\left(2.5\right)\left(4\pi \right)$
$=10\pi$
$=31.4kg{m}^2{s}^{-1}$