817 Hollow Tube | Moment of Inertia and Radius of Gyration

Polar moment of inertia
$\bar{J} = \frac{1}{2}\pi(R^4 - r^4)$

$\bar{J} = \frac{1}{2}\pi(3^4 - 2^4)$

$\bar{J} = 32.5\pi \, \text{ in.}^4 = 102.10 \, \text{ in.}^4$           answer
 

Area
$A = \pi(R^2 - r^2)$

$A = \pi(3^2 - 2^2)$

$A = 5\pi \, \text{ in.}^2$
 

Radius of gyration
$\bar{k}_z = \sqrt{\dfrac{\bar{J}}{A}}$

$\bar{k}_z = \sqrt{\dfrac{32.5\pi}{5\pi}}$

$\bar{k}_z = 2.55 \, \text{ in.}$           answer