Hi everyone!! Please help, I have spent some considerable time to understand the two concepts and still this is nagging at me.... I am relating to Structural Engineering, just to let you know guys. My question is .. Moment of inertia is about distribution of mass, the further away from the axis the higher the resistance to applied moment ( I believe I know the concepts of angular acceleration and etc.). Likewise in case of Elastic Section Modulus if the distribution of area of section is larger away from the axis the resistance to elastic deformation is greater... So in terms of Structural Engineering what is the difference between the two? I know that moment of inertia is related to mass (althou it is in cm^4 which is brings another question, why is it not in kg?), but as in practical sense why do we need one or the other to determine the strength of the beam, lets say up to elastic limit???