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i need to find the equivalent spring constant K of a shaft with radius=r, length=l, and modulus of rigidity=G

I know that K=GJ/l

Question 1) Is J in this case the AREA moment of inertia? I think it is.

Question 2) Why can't I find online a formula for the J of a shaft?

I know the cross section is simply a circle and it is rotating about

a point passing through its center and perpendicular to the plane of the circle

I do not have a mechanics book with me (at work). Can anyone help me out here?

Thanks!