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Light speed

  1. May 20, 2006 #1
    If we have two airships going at speed 0.6c in the opposite directions. then they will be seperated at a speed higher than light.

    let's name the airships , A and B
    My question is , Let's say we have the two airships pull an elastic material.
    take a point of the material on Airship A name it a , and a point on airship B. name it b. if they go at a speed close to light , then point a and b will be going at a speed faster than light ?

    If so. then shouldn't all the points on the elastic material be going at a speed faster than light ?
    Last edited: May 20, 2006
  2. jcsd
  3. May 20, 2006 #2
    You don't add relativistic speeds in the same way you do with everyday speeds.

    [tex]V = \frac{v+u}{1+\frac{uv}{c^{2}}}[/tex]

    Let v = 0.6c and u = 0.6c, that gives V = 0.88c, so they aren't moving apart at a speed greater than light, but less than light.
  4. May 20, 2006 #3


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    Staff: Mentor

    If the spacecraft are going .6C, then quite obviously, the fastest any part of the material will be going is .6C - right where it is connected to a spacecraft.

    Also quite obviously, a point at the center of an elastic band is stationary wrt where the stretching started. Try it with a broken rubber band - put a mark at the center and pull the ends apart. The mark doesn't ever move.
  5. May 20, 2006 #4
    Thank you.
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