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: Help with relativistic momentum question

  1. Dec 23, 2012 #1
    1. The problem statement, all variables and given/known data

    a D meson is at rest and decays into a Kaon and a Pion. The Kaon moves with speed 0.867c and has a mass of 0.494 GeV/C^2. The pion has a mass of 0.140 GeV/C^2. use conservation of momentum to calculate the speed of the Pion.

    2. Relevant equations
    Relativistic Momentum P = [itex]\gamma[/itex]mV

    where [itex]\gamma[/itex] is [itex]\frac{1}{\sqrt{1 -\frac{v^{2}}{c^{2}}}}[/itex]

    3. The attempt at a solution

    So if the D meson is initally at rest, initial momentum = 0, which means

    [itex]\gamma[/itex][itex]_{v1}[/itex]m[itex]_{1}[/itex]v[itex]_{1}[/itex] = [itex]\gamma[/itex][itex]_{v2}[/itex]m[itex]_{2}[/itex]v[itex]_{2}[/itex]

    Where particle 1 is the Kaon and particle 2 is the Pion, we want the speed of Pion so we solve for v[itex]_{2}[/itex]

    After some rearrangement I got v[itex]_{2}[/itex][itex]^{2}[/itex] = [itex]\frac{1} {\frac{m_{2}^{2}}{(\gamma_{v1}m_{1}v_{1})^{2}} + \frac{1}{c^{2}}}[/itex]

    After plugging in the numbers m2[itex]^{2}[/itex] = ([itex]\frac{0.140x10^{9}}{(3x10^{8})^{2}}[/itex])[itex]^{2}[/itex]

    and m1[itex]^{2}[/itex] = ([itex]\frac{0.494x10^{9}}{(3x10^{8})^{2}}[/itex])[itex]^{2}[/itex]

    and [itex]\gamma[/itex][itex]_{v1}[/itex] = [itex]\frac{1}{\sqrt{1 - 0.867^{2}}}[/itex]
    I get an answer faster than light, where have I gone wrong?
  2. jcsd
  3. Dec 23, 2012 #2


    User Avatar
    2017 Award

    Staff: Mentor

    I would expect that your rearrangement is wrong. Even if you use wrong numbers for the masses, the speed has to be below the speed of light in every relativistic calculation.

    Edit: After a closer look at your equation, v2 calculated there should always be below c. The formula might be right, but then your evaluation is wrong.
  4. Dec 24, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    In special relativity problems, you'll find it generally better to stick to working with energy and momentum rather than velocities as it simplifies the algebra quite a bit. Try finding E and p for the pion. Once you have those, you can find its speed using the relation v/c = pc/E.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook