Until now, I always that that a relativistic mass increase was literally just the increase in the mass of an object. Today I googled it, and a couple sources say that it is just an increase in energy, not mass. Can anyone confirm this? At first I thought if the mass increased, then it would take more energy to accelerate that mass, and eventually it would take infinite energy. But if its really energy that increases, then this logic doesn't make sense. Unless it still takes more energy to accelerate a particle with a higher energy? Finally, why does the energy increase?