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

Relativity Question: Two spaceships following each other near C

  1. Aug 13, 2004 #1

    What equations do I use to calculate how frame A observes frame B's velocity and vice versa if two spacecraft are following each other?

    is it:

    v = ux + v / ( 1 + (Ux*v)/(c^2))

    I have a feeling it isnt since if both craft are travelling say, 2.5*10^8 m/s, then the observed speed from another frame is .98c off the top of my head. I know this is relativity and all, but shouldnt frame A perceive frame B as simply going 0 m/s?
  2. jcsd
  3. Aug 13, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That is correct, if the 2 ships would have the same velocity then you do not need to use the equations of SR. Note that there must be some reference frame from which they measure a velocity of .95c. That is the frame which must use relativity calculate the ships speed and distance traveled.
  4. Aug 13, 2004 #3

    Now lets say ship A is traveling .6c and ship B is traveling .8c, a would view b speed as...

    v = ux - v / ( 1-(ux*v)/(c^2))


    v = (.8c - .6c) / ( 1- (.6c*.8c)/(c^2) = .34c

    so ship A view ships B as travelling .34c faster?
  5. Aug 13, 2004 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Assuming you mean that A travels at 0.6c and B travels a 0.8c with respect to some common frame.
    Closer to 0.38c
    Ship A sees ship B moving at a speed of 0.38c with respect to ship A. And vice versa.
  6. Aug 13, 2004 #5
    [tex] u' = \frac {u+v}{1- \frac{uv}{c^2}} [/tex]
  7. Aug 13, 2004 #6
    Thanks Everyone :)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?