# A regulation table tennis ball is a thin spherical shell

A regulation table tennis ball is a thin spherical shell 40 mm in diameter with a mass of 2.7 g. What is its moment of inertia about an axis that passes through its center?
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kalfswors0m
The moment of inertia of the ball you can find using equality:
$I=\frac{2}{3}m{r}^{2}$
$I$ is the inertia of the tennis ball, $m$ is the mass of the ball, $r$ is a rafius of the ball.
$d=40mm$, $m=2.7g$
$r=\frac{1}{2}d=\frac{1}{2}×40×{10}^{-3}m=0.02m$
$m=2.7g×{10}^{-3}kg=0.0027kg$
Substitute the values to the formula:
$I=\frac{2}{3}×0.0027kg×{\left(0.02m\right)}^{2}=7.2×{10}^{-7}kg×{m}^{2}$

xandir307dc
Good job, thanks!