# Mass increasing with energy

I've heard that if you heat up a substance it masses very slightly more, because of E = mc2, and the ebergy added is in the form of heat
I am just curious - could you also think of it as m = m0 / $$\sqrt{1 - v^2 / c^2}$$ because heat energy is just really diffuse kinetic energy on a molecular scale, and the small sums of all of the masses added is measurable given extremely precise instruments?

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Drakkith
Staff Emeritus
I would assume you are correct, but remember that having more mass doesn't change the rest mass, or invariant mass, or whatever you want to call it.

I would assume you are correct, but remember that having more mass doesn't change the rest mass, or invariant mass, or whatever you want to call it.
Right, but rest mass is the mass measured by an object also moving at the same velocity, which is impossible when ever molecule has a different velocity vector.

bcrowell
Staff Emeritus
Gold Member
I would assume you are correct, but remember that having more mass doesn't change the rest mass, or invariant mass, or whatever you want to call it.
If you add heat to a body, you *do* increase its rest mass.

To take the simplest nontrivial case, suppose you have a system of two particles, each of rest mass m, moving in opposite directions at speed v. Taken as a single unit, these two particles have a rest mass of more than 2m.

Drakkith
Staff Emeritus