I'm very noob at this and am a bit confused: Formula 1: [itex] E_T = \gamma \cdot m c^2 [/itex] Formula 2: [itex] p = \gamma m v[/itex] Formula 3: [itex] E_T^2 = (pc)^2 + (mc^2)^2[/itex] Formula 3 says a particle of negligible mass can have energy, but isn't this in contradiction to formula 1? Unless maybe the velocity of the particle is c, such that ##\gamma## becomes infinite too? So if I get an assignment where I'm supposed to neglect the mass of a moving particle, I must either neglect its total energy or set its velocity to c in my calculations? For instance, let's say I need to calculate on myon decay: ##\pi_+ \rightarrow \mu_+ + \nu##. Do I have to set the velocity of the myon-neutrino to c? But why can't I just neglect the energy of the neutrino?