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If two spaceships are moving toward each other at the 0,9c

  1. Sep 25, 2011 #1

    I wonder, if we have two spaceship and they are moving tovard each other at the same 0,9c speed. what is the relative speed as seen in the other ship?

    if our referance is another motionless ship. i think, ship's 0,9c speed seen ~0,4c and we're seeing they're closing each other, 0,4c speed. (or, am i wrong?)

    but, we're sitting the travelling ship. so, what is the other ship's relative speed?

    (sorry for my bad english)

  2. jcsd
  3. Sep 25, 2011 #2


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    Whatever speed the first ship sees or measures of the second ship is exactly the same speed the second ship will see or measure of the first ship. They can be each other's reference, you don't need a third ship to serve that purpose.
  4. Sep 25, 2011 #3

    i think i found the right equation

    v2 = -0,9c
    v1 = +0,9c

    v2' = (v2-v1) / (1-(v1*v2/c²)

    v2' = 0,994475... c

    is that right?
  5. Sep 25, 2011 #4


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    That's the how fast each of the first two spaceships will approach each other if they are both approaching a third spaceship at 90%c.
  6. Sep 25, 2011 #5
    As a summary (it has been more or less answered already):

    - If you see two spaceships moving toward each other, each at a speed (according to you) of 0.9c, then you will measure their closing speed to be 1.8c.
    It cannot be otherwise (you can easily check for yourself that this a mathematical necessity).

    - The speed with which one spaceship (with its independent measurement system) will measure the other spaceship coming closer:
    1.8c / (1 + 0.81) = 0.9945 c

    See also http://en.wikipedia.org/wiki/Velocity-addition_formula


    PS welcome to physicsforums:smile:
    Last edited: Sep 25, 2011
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