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 in Special Relativity

  1. Nov 23, 2012 #1
    Hi All,

    Pardon me if this question looks so silly.

    Trying to understand velocity addition in special relativity.

    Say velocity components as measured in stationary frame of reference S are u, v, w in x, y, Z directions respectively and those in moving frame S' are u', v', w' in x', y', z' directions respectively. Let the velocity of relative motion between the reference frames is V and the motion is along x(or x') direction. Then velocity addition equations are as follows

    u = (u'+V)/(1+(u'V/c^2)) -----(i)

    v = {v'[1-(v^2/c^2)]^(1/2)}/[1+(u'V/c^2)] -----(ii)

    w = {w'[1-(v^2/c^2)]^(1/2)}/[1+(u'V/c^2)] -----(iii)

    Now say if light is emitted in the moving frame S' in its direction of motion x' i.e u'=c, then an observer in S measure the speed as u=c according to the equation (i)

    But how do I check using eqn (ii) that light emitted in y' direction in frame S' has speed c in frame S. I substitute u'=0 and v'=c in eqn (ii), but that leads to v=c[1-(v^2/c^2)]^(1/2)

    Please help me in figuring out where I go wrong
     
  2. jcsd
  3. Nov 23, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Don't forget equation (i): The light will have a component of velocity in the x direction.
     
  4. Nov 23, 2012 #3
    Thanks for pointing out, Doc
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook