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

Velocity addition

  1. Oct 31, 2009 #1
    I know how to add speed in special relativity.


    Is this a vector thus velocity? Or does this only apply to objects traveling on the same axis? Judging from the way it's derived, I really think that it only applies to objects on the same axis. What if I want to look at say...an object going north and an object going east in the rest frame. What then is their velocity in the frame of one of the objects?
  2. jcsd
  3. Oct 31, 2009 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    It's only for motion along one axis. You can check this yourself: if you consider the non-relativistic limit, you only get the right answer for motion along one axis.
  4. Oct 31, 2009 #3

    Doc Al

    User Avatar

    Staff: Mentor

    As you realize, the formula you quoted (once you correct the sign error in the denominator) is only for the special case of parallel velocities. Read about the more general case here: http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html" [Broken]
    Last edited by a moderator: May 4, 2017
  5. Oct 31, 2009 #4


    User Avatar
    Science Advisor

    You should have a plus sign in the denominator.
    There is a different formula for u and v perpendicular.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook