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Speed of light questions

  1. Oct 5, 2008 #1
    1. The problem statement, all variables and given/known data

    A signal (traveling at the speed of light) is sent from -4 ls (light seconds) towards a moving spaceship traveling at 0.866c. How long does it take for the signal to reach the ship?

    2. Relevant equations
    I'm pretty sure its t=(delta)D/c-v

    But I continue to get the wrong answer with it.


    3. The attempt at a solution

    using the equation above [rearranged from d(light) = (delta)d + vt]. I get 29 seconds. I do it by multiplying 4 ls by 3e8 and use that as my distance.

    according to a spacetime diagram I have, the answer should be around 12s (my diagram might be wrong, but it doesn't seem so). But I can't seem to get it with this equation.
     
  2. jcsd
  3. Oct 5, 2008 #2

    Doc Al

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    Staff: Mentor

    Which way is the ship moving? Where is it at t = 0?
     
  4. Oct 5, 2008 #3
    the ship is moving in the positive direct and is at 0 ls when t = 0
     
  5. Oct 5, 2008 #4
    btw, this is all from signal sender's frame of reference.
     
  6. Oct 5, 2008 #5

    Doc Al

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    Staff: Mentor

    Why do you think your answer is wrong?
    I get a slightly different answer.
     
  7. Oct 5, 2008 #6
    hm... becuase I have a spacetime diagram modeling this answer and it seems that it is only supposed to be 12 seconds....
     
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