1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 4-momentum of particles in lab and CM frame

  1. Apr 13, 2012 #1
    The question:
    Suppose two identical particles, each with mass m and kinetic energy T, collide head-on. What is the kinetic energy of one in the rest system of the other?

    The solution:

    Is given on pg 109 of Griffiths' Introduction to Elementary Particles.

    Griffiths writes down the total 4-momenta of the system in the lab frame and in the CM frame.

    The zeroth component in the CM frame is 2E/c. So I guess that E is the energy of one of the particles in that frame.

    The zeroth component in the lab frame is E'+mc^2, however. Why is mc^2 added for the lab frame? Why not have 2E'/c as in the CM frame?

    Thank you!
  2. jcsd
  3. Apr 13, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The mc^2 is the energy of the particle that is at rest. E' is the energy of the particle that's moving.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook