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: Electrodynamics and Relativity. Please Help

  1. Mar 3, 2012 #1
    Electrodynamics and Relativity. Please Help!!!!

    1. The problem statement, all variables and given/known data

    Consider the decay of a particle.

    (a) Find the threshold kinetic energies of the incident particles in each of
    the following reactions:

    (i) ∏+ + p →∏++ ∏° + p
    (ii) p + n →p + n + ∏° + ∏°

    The incident particle is the first particle listed in the reactions; the target particle is at rest. The masses of the ∏+ and ∏° ions are 139.567MeV/c2 and 134.963 MeV/c2 respectively while the masses of the neutron and proton are respectively 939.566 MeV/c2 and 938.272 MeV/c2.

    2. Relevant equations

    The teacher suggested that I will need to use the following equations to get the solution
    (Here P is the momentum and Px is the momentum of the incident particle ∏+in the X direction, assuming it hits the target particle p(proton) horizontally, so no P(momentum) in y or z direction)


    PuPu=m2c2= E2/C2-P2

    Also said that using this notations will be useful to solve the problem

    For ∏+, the incident particle:
    Pu=(E/C, Px, 0, 0)
    Pu=(E/C, -Px, 0, 0) where Px is the momentum of the incident particle, and no need to worry about y and z direction so just put zeros.

    3. The attempt at a solution
    This is what I have done so far to solve for part i, but I have no clue how to continue.
    Since we are solving for the threshold kinetic energy, so assuming that ∏+, ∏° and p are at rest after the reaction.

    (E/C, Px, 0, 0) +(mpC2, 0, 0, 0) - (m+C2,0, 0, 0 ) -(m∏°C2,0, 0, 0 ) - (mpC2,0, 0, 0 ) = 0

    mp, m+, m∏° are just the rest mass.

    PLEASE PLEASE help me to do this problem!!! thank you so much!
  2. jcsd
  3. Mar 3, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: Electrodynamics and Relativity. Please Help!!!!

    As you see, trying to have all 3 final state particles at rest is impossible due to conservation of momentum. You need to allow for some final momenta and then think about what being at threshold implies.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook