Okay, maybe I am retarded, but i am having trouble with this. 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!