Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Relativistic Momentum Help With Equation Reduction

  1. Aug 17, 2011 #1
    Hi, so basically have been looking at http://en.wikibooks.org/wiki/Special_Relativity:_Dynamics#Momentum" and working my way through the maths for myself. However I have hit this point and can't get past it:

    u = \frac{v - u}{1-\frac{uv}{c^2}}
    Which should be able to become:
    u = \frac{c^2}{v(1-\sqrt{1-\frac{v^2}{c^2}})}
    I have tried and tried but can't seem to get it to work. Can anyone help me out on this one please?!

    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Aug 17, 2011 #2


    User Avatar
    Homework Helper
    Gold Member

    That wiki entry is not being clear with it's order of operations!

    What you should be getting is:

    [tex] u = \frac{c^2}{v}(1-\sqrt{1-\frac{v^2}{c^2}})[/tex]
    Last edited by a moderator: Apr 26, 2017
  4. Aug 17, 2011 #3
    Hi G01, thanks for your reply. Have tried aiming for that equation instead and still can't get there. I have tried all sorts of different approaches, such as getting it in the form of a quadratic or dividing the top and bottom of the original fraction by c^2, but I just end up nowhere. Could you perhaps give me a hand in the right direction, maybe the first step or two - but don't make it too easy for me!
  5. Aug 17, 2011 #4


    User Avatar

    Staff: Mentor

    Show us exactly what you've tried, and where you get stuck, and tell us why you're stuck.
  6. Aug 17, 2011 #5
    Ok, have shut my computer down now and am replying on my phone. I will post my attempts tomorrow.
  7. Aug 17, 2011 #6


    User Avatar
    Science Advisor
    Gold Member

    Well, brute force use of the quadratic formula leads directly to the desired answer.
  8. Aug 19, 2011 #7
    Haha, went back over my general quadratic attempt after PAllen's suggestion and realised that I had made a rather fundamental cross multiplication error! Fixed that and got to the desired equation. Lovely!

    Last question, the equation that I get is:
    u = \frac{c^2}{v}(1\pm\sqrt{1-\frac{v^2}{c^2}})
    I assume that we choose the negative version of the equation, because the positive version would yield a value for u which is greater than c?

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook