Probably Pmb won't agree with me, but mass doesn't change with velocity (of course I'm referring to invariant = proper mass).

Einsteins famours equation e=mc^2 says that as velocity increases, so does mass. But you just said mass doesn't change with velocity. Can you explain the conflict?

What is mass? How do you define it? What does that equation really mean?

In special relativity (that is, in a flat spacetime) there are just two kinds of mass: proper = invariant mass and relativistic mass. The second, however, is much less used now than in the past, because of various reasons. We are left with the first one, invariant mass (when physicist says "mass" they almost always mean that one).

Then, the correct equation which relates mass to energy is not the one you wrote, but:

E = Sqrt[(cp)

^{2} + (mc

^{2})

^{2}]

E = energy

m = mass

p = momentum

the one you wrote is then valid *only* at zero momentum, for example for a still body.

The term "invariant" means, in relativity "it doesn't change from a frame of reference to another one". Invariant mass is one of such quantities, so it doesn't change with speed. An electron has a mass of ≈ 9*10

^{-31} kg, and it keeps that value whatever its speed (and the same for anything else).

What changes with speed is momentum and energy, not mass.