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Special relativity: firecracker explode simulatenously, distance of observation?

  1. Nov 2, 2009 #1
    I have a simple question.... say two firecrackers explode simultaneously, and there's an observer in the same reference frame at rest. The observer is standing closer to one firecracker, but since she is at rest, does she observe them to explode simultaneously?
    If not.... I cannot find the equation I need to use to find the different times the light of the explosion reaches her eyes. (say she is standing 600 m from firecracker A, whose light reaches her eyes in 3.0 mu-s. and she is standing 300m from firecracker B).
    Thanks =)
     
  2. jcsd
  3. Nov 2, 2009 #2

    Delphi51

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    Re: special relativity: firecracker explode simulatenously, distance of observation..

    Both fire crackers and the observer are at rest relative to each other?
    Then you don't use relativity. Just d = vt for the motion of the light.
     
  4. Nov 3, 2009 #3
    Re: special relativity: firecracker explode simulatenously, distance of observation..

    I thought so...

    But I am really confused now.
    The values in the problem is that a person is standing at x = 600m. firecracker 1 is at the origin, and firecracker 2 is at x = 900m. the firecrackers explode simultaneously. it is given that the light from firecracker one reaches the person's eyes in 3 microseconds. What time does she see the flash from firecracker 2?

    The answer in the back of the book gives 2 microseconds. this makes no sense to me no matter which way i approach the problem. any help?
    thanks
     
  5. Nov 4, 2009 #4

    Delphi51

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    Re: special relativity: firecracker explode simulatenously, distance of observation..

    I can't think of a way to get that answer. I tried moving the observer 671 m away from the x axis to get that 3 microseconds using the regular value for the speed of light. This gives 2.45 microseconds for the time the flash from the 2nd firecracker being seen. It matches the answer to 1 digit accuracy.

    Have you precisely copied the question?
     
  6. Nov 4, 2009 #5
    Re: special relativity: firecracker explode simulatenously, distance of observation..

    Girl is standing at x = 600m. firecracker 1, at origin, and firecracker 2 at x = 900m, explode simultaneously. the flash from firecracker 1 reach's girls eyes at t = 3.0 microseconds. at what time does she see the flash from firecracker 2?

    im not even sure the correct way to approach the problem. when i use t = d/v to check the given number... its not even correct.... 600m/3x10^8 gives me 2 microseconds, not 3. the back of the book gives me an answer of 2 microseconds for the time firecracker 2 reaches her eyes.

    error possibly?
     
  7. Nov 4, 2009 #6

    Delphi51

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    Re: special relativity: firecracker explode simulatenously, distance of observation..

    Either that or we are missing something.
     
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