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Homework Help: Train and Speed of Sound in Air versus Track

  1. May 2, 2009 #1
    1. The problem statement, all variables and given/known data

    Stan and Ollie are standing next to a train track. Stan puts his ear to the steel track to hear the train coming. He hears the sound of the train whistle through the track 3.1 s before Ollie hears it through the air. How far away is the train? (Use 5,790 m/s as the speed of sound in steel and 343 m/s as the speed of sound in air.)

    2. Relevant equations

    distance = velocity*time
    v-steel= 5,790m/s
    v-air= 343m/s

    3. The attempt at a solution

    distance-stan = 3.1seconds*5790m/s = 17,949m
    distance-olie = 3.1seconds*343 = 1063.5m

    Not sure what to do from here. I don't quite understand the concept relationship they're trying to establish...

    I tried subtracting Olie's distance from Stan's, but that didn't get me the right answer...
  2. jcsd
  3. May 2, 2009 #2
    The assumption you need to make is that Stan and Ollie are standing next to each other. ie the distance is the same.

    Then you can set up the equation: v(stan) x t(stan) = v(ollie) x t(ollie)

    Remember that the question states that Ollie hears the sound 3.1 seconds after Stan, so t(ollie) = t(stan) + 3.1

    Substitute this in, expand brackets, solve for t(stan).

    Then use d = vt for this time and speed of sound in steel.

    Hope this helps.
  4. May 2, 2009 #3
    I end up with this:

    T(stan) = (V(olie) x (T(stan)+3.1s)) / V(stan)

    I'm not quite sure how to solve...
  5. May 2, 2009 #4
    Leave t(stan)*v(stan) = v(ollie)*(t(stan) + 3.1)

    Now expand the brackets.

    Then get the t(stan) factors on the same side of the equals sign.

    Then factorise

    Keep trying! :)
  6. May 3, 2009 #5
    Now I end up with something like this:

    Ts + VoTs = 3.1Vo / Vs

    I don't know how to get the Ts's alone...
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