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Homework Help: Help: Length Contraction and changing Frames

  1. Mar 31, 2008 #1
    [SOLVED] Help: Length Contraction and changing Frames

    Hi i have been learning about length contraction and i thought i understand how it works until i was given this question.
    A very fast train of proper length Lo rushes through a station which has a plaform of length L(<Lo).
    According to this observer, two porters standing at either end of the platform (distance L apart) kick the train simultaneously, which makes dents in it. When the train stops, the dents are distance Lo apart.
    How is the difference between L and Lo explained by:
    (a)the observer on the platform?
    (b)the observer traveling on the train?

    My attempt at a solution
    (I have no idea what happens when something is being lenght contracted in one frame and then goes to another frame where everything is at rest.)
    (a)Since the train is no longer under length contraction when it has stopped the observer on the platform will say that the train has expanded in the x direction. To explian the difference between Lo and L.
    (b)However the observer traveling on the train will see something different. Since this is in a different frame we know that the two porters will not kick the train simultaneously. also we know because of time dilation that the two porters will have slower clockers than trains clock. making the dents at Lo distance instead of L distance.

    Hopefully, someone can help me out with this. Cheers.
  2. jcsd
  3. Mar 31, 2008 #2


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    I think you have the essential point. In one frame the kicks are simultaneous, in the other frame they aren't. If you really want to learn this try putting actual numbers in. Put the kickers in at t=0 and x=0 and x=1 in the rest frame. Pick a value of v and figure out what goes on the the moving frame. Both scenarios make perfect sense.
  4. Apr 2, 2008 #3
    Okay, thanks for the reply. I just wasn't to sure what observer (a) would see. If the train would look like it has expanded in length. Its a little hard to comprehended sometimes.
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