1. Not finding help here? Sign up for a free 30min 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!

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?