If you have a particle, and you know its rest mass is about 100 Mev/c^2 and in the lab frame you measure it to travel at (8/9)^1/2 c then what is its energy in the rest frame?? Would it be just 100MeV? or 300 MeV due to the relativistic constant? i don't really know how you would reason out the difference in frames? can anyone explain this idea?