I've just started an introductory modern physics course, and it's making me ache a bit. I'm unsure about this problem: So, if I calculate the momentum of the two pi mesons in the rest frame of the decaying meson... well... I've gotten that far. I'm not sure if I need to transform the momentum somehow back into the labframe. I know I can transform the velocity, but that doesn't seem useful. And then I have some inkling that the different velocities depend on the direction of the resulting mesons, but I'm not sure what laws to use to obtain them. It's also not clear to me whether the different speeds are only to be different because they're being measured from the labframe or because they're actually different. I'd appreciate any advice/help that could be given.