The polar moment of inertia of a hollow circular cross-section is given by the formula:
$J = \dfrac{\pi}{32} (D_o^4 - D_i^4)$
where $D_o$ is the outer diameter and $D_i$ is the inner diameter of the jacket. In this case, $D_o = 80 \,\text{mm}$ and $D_i = 80
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