Chapter 6: Q2E (page 1247)
The positive muon , an unstable particle, lives on average (measured in its own frame of reference) before decaying. (a) If such a particle is moving, with respect to the laboratory, with a speed of 0.900c, what average lifetime is measured in the laboratory? (b) What average distance, measured in the laboratory, does the particle move before decaying?
Short Answer
The total rest mass of the system is the sum of the relativistic masses of the particles, so in theory, a hot gas should have more mass than a cold one, but the difference is negligible.