1. The problem statement, all variables and given/known data pi-meson's rest energy is 139.57MeV, it decays into a muon and a neutrino. The muon has a rest energy of 105.45MeV, and a proper lifetime of 2.197*10^-6. The neutrino's mass can be treated as massless in the process. 1) Assuming the pi-meson decays at rest, what is the momentum of the muon? 2. Relevant equations E_muon + E_neutrino = E_pi-meson E_muon = c^2* ((m_pi-meson)^2 - (m_muon)^2)) / (2m_pi-meson) E_muon = sqrt((P_muon^2 c^2) + (m_muon c^2)^2) 3. The attempt at a solution Ok I used used the second equation and it gives about 9.866*10^18, then, use this value in the 3rd equation and solving for the momentum, I get a negative value under the square root, which doesn't make sense. In the 2nd equation, the values I plugged in for m_pi-meson and m_muon are just 139.57 and 105.47, and used 3*10^8 for c. Where did I go wrong?