You can use polar moment of inertia to calculate the rotational inertia of an object with constant cross-section:

 where I is the rotational inertia of the object,
  is the polar moment of inertia of the cross-section of the object,
  is the density of the material, 
 and l is the length of the object.
 Unlike the torsion application, this is true for every cross-sectional shape, even non-circular shapes.
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