Which of the two is stiffer in bending? 1. A circular rod of diameter D and length L. 2. A circular rod of diameter d and length L, surrounded by a tube of inner diameter d, outer diameter D, and length L. The tube is not bonded and can freely move. It seems like an easy solution in that we only need to calculate the second moments of area, which yields the same stiffness for both cases. However, I've had a hard time convincing people of this and would like to confirm that the bending stiffnesses are, indeed, equal. Thank you!