My textbook states that translational KE and rotational KE are completely analogous. The author states "They both are the energy of motion involved with the coordinated(non random) movement of mass relative to some reference frame. I can understand why this applies when comparing rotational velocity and translational velocity, but how can we justify that mass is equal to moment of inertia? It appears to me that moment of inertia isn't exactly analogous to mass in that it involves both direction and a physical quantity.