- #36

Slimy0233

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hey, thank god you had commented this, btw, my professor used ##E = mc^2## to calculate the velocity of a moving electron instead of the general expression ## m^2 c^2 = E^2/c^2 - p^2##, so is he wrong here? Third line (I mean, I think he is wrong, but he is my tutor, so, I sought your helpDale said:Nowhere. There is nowhere that you should use relativistic mass.Everywhere. You should avoid using relativistic mass everywhere.No. The ##m## in ##E=mc^2## is the invariant mass since the formula only applies to systems at rest anyway. Although it is super-famous, ##E=m c^2## is not a very general expression. The general expression is $$ m^2 c^2 = E^2/c^2 - p^2$$ The famous equation is the special case of the general equation for ##p=0##.

So, the famous equation is only true when the momentum of the system is 0. Thus the ##m## in the famous equation is the invariant or "rest" mass. This ##m## is also the ##m## that you find in tables listing the properties of particles, the ##m## that you use in the relativistic version of Newton's 2nd law, and the ##m## that you measure with a balance scale.

edit: The kinetic energy of the electron is 0.34 MeV and thus the total energy will be 0.51MeV+0.34MeV = 0.85 MeVSo, I would say ##({(0.85Mev)}^2 = (mc^2)^2 + (pc)^2## and not the special case

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