> And which kind of mass is a proxy for the total energy of a system? When considering systems there is only one mass. It's called mass. It's neither relativis

> And which kind of mass is a proxy for the total energy of a system? When considering systems there is only one mass. It's called mass. It's neither relativistic nor rest mass. It's just E/c2 where E is the total energy of the system. It's not the sum of the parts because it's dependent on interactions between the parts as well. The mass of deuterium nucleus is not the sum of relativistic masses of the proton and neutron. It's less because they are bound. Rest mass is a sort of useful concept because it is the bit of energy that's still there even when object is at rest. Apparently it has something to do with Higgs field which I'm not super keen on because I don't understand it but nothing seems to contradict this so far. But talking about relativistic mass doesn't make much sense. The velocities are involved in mass only because equation for the energy of a particle has momentum term. It doesn't create anything qualitatively different to be worth naming it. Relativistic mass is only talked about to avoid contradicting the intuition of mass being quantity of matter which was introduced earlier to weed out the intuition of mass being weight which kids get from everyday language. It's really not a great idea because it misleads people who try to think for themselves a bit further. For example what would happen if you tried to push fast moving object sideways. Would it be harder because of higher innertia because of higher relativistic mass? After all trying to push it in the direction it travels should be harder because of high relativistic mass. That's what people say when trying to explain why you can't accelerate matter to the speed of light. Suddenly relativistic mass becomes a directional property. When you use the correct equations in their full form istead of trying to shortcut to skip momentum and vectors all of this confusion disappears.