Question

A uniform circular disc of mass $200 g$ and radius $5 cm$ is rotated about one of its diameters at an angular speed of $15\,rad / sec$ What is the angular momentum about the axis of rotation?

• A. $16 \,Js$
• B. $18.75 \,Js$
• C. $19 \,Js$
• D. $22 \,Js$

Answer 1

Answer: (B)

Moment of inertia of disc about the diameter
$I=\frac{M R^{2}}{4}$
$=\frac{1}{4} \times 200 \times 10^{-3} \times\left(5 \times 10^{-2}\right)^{2}$
$I=\frac{0.2 \times 25 \times 10^{-4}}{4}=\frac{5 \times 10^{-4}}{4}$ (Given $\left.\omega=15 rad / s \right)$
$\therefore L=I \omega=\frac{5 \times 10^{-4}}{4} \times 15=\frac{75}{4} \times 10^{-4}$
$=18.75 \times 10^{-4} J - s$