Hello, In deriving the energy-momentum equation: [tex] E^2 = (pc)^2 + (mc^2)^2 [/tex] the following equations are used: [tex] p = ymv [/tex] [tex] E = ymc^2 [/tex] But both equations are equations that depend on mass, while the final result does not and applies to massless particles. Besides the energy-momentum equation is often cited as the equation relating mass and relativity rather than the one from which it was derived. This leads me to think that this equation is more fundamental than either of the ones used to derive it, so there should be a way to derive it without these two. However, I cannot find any such method. Does anyone knows of how I could derive it without using those two equations? Thank you.