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Homework Help: Two explosions occur, spaceship is flying overhead at 0.6C, which occurs first?

  1. Sep 13, 2012 #1
    1. The problem statement, all variables and given/known data
    At 11h 0m 0.0000s AM a boiler explodes in the basement of the Denver Science Museum. At 11h 0m 0.0003s, a similar boiler explodes in the basement of a ski lodge in Aspen at a distance of 150 km from the first explosion. Show that in the reference frame of a spaceship moving at a speed greater than v=0.6c from Denver to Aspen, the first explosion occurs after the second.

    2. Relevant equations
    Lorentz Transforms
    x=[itex]\gamma[/itex](x' + vt')
    t=[itex]\gamma[/itex](t' + vx'/c^2)

    Δt = [itex]\gamma[/itex]vL/c^2
    time and length dilation

    3. The attempt at a solution
    I first state that from the ship's perspective, the two boilers are moving, the one in denver away from the ship at 0.6c, the one in aspen towards the ship at 0.6c. I can find that, due to time dilation, the actual time between explosions as viewed from the ship would be 0.00024s, but dont know where to go from there. Also I used the simultaneity equation using Δt=0.0003s to find the distance between the two blasts, which was just length dilation in the end so i did more work for nothing. Im stumped now though. I have both dilated time and length of travel, but how can i show that the explosion in aspen happens first?
  2. jcsd
  3. Sep 13, 2012 #2


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    Welcome to PF Mrbilly!

    All you need to know to solve this problem is that the spacetime coordinates of events in two different coordinate systems (reference frames) are related to each other by the Lorentz transformation. You have x1 and t1, which are the location and time of the first explosion in the unprimed (Earth) reference frame. You also have x2 and t2, which are the location and time of the second explosion in the Earth frame. All you have to do is apply the Lorentz transformation to each pair of coordinates (x,t), to find x1ʹ, t1ʹ, x2ʹ and t2ʹ, which are the spacetime coordinates of the two explosions in the primed (ship) reference frame. You should find that t1ʹ > t2ʹ.

    Note: this belongs in the Introductory Physics subforum, since the Advanced Physics one is for upper-year undergraduate and graduate-level physics homework only. Thread moved.
  4. Sep 13, 2012 #3
    Thanks for moving the thread, sorry about that.

    I did all the math, but i still come up with the time for the denver explosion at 0 seconds and the aspen explosion at somehow a longer time of 0.000375s. Am I wrong in saying that the (x,t) coordinates of the denver boiler are (0,0) and aspen is (150,0.0003)? just because with the (0,0) coordinates, everything will just go back to being 0...
  5. Sep 13, 2012 #4


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    No you're not wrong. Those two sets of coordinates are correct. Remember that you're going from the unprimed (Earth) coordinate system to the primed (ship) coordinate system, so the transformation equations are:

    xʹ = γ(x - vt)
    tʹ = γ(t - vx/c2)

    What you wrote down in your first post was the reverse transformation from this (going from primed to unprimed).
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