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Special Relativity of Train Problem

  1. May 19, 2016 #1
    1. The problem statement, all variables and given/known data
    A relativistic train of proper length 237 m approaches a tunnel of the same proper length, at a relative speed of 0.951c. A paint bomb in the engine room is set to explode (and cover everyone with blue paint) when the front of the train passes the far end of the tunnel (event FF). However, when the rear car passes the near end of the tunnel (event RN), a device in that car is set to send a signal to the engine room to deactivate the bomb. Train view:(a) What is the tunnel length? ans: 77.3m (b) Which event occurs first, FF or RN? ans: FF (c) What is the time between those events? ans: 5.74E-7 s (d) Does the paint bomb explode? ans: YES Tunnel view:(e) What is the train length? ans: 77.3m (f) Which event occurs first? RN (g) What is the time between those events? ans: 5.74E-9 s (h) Does the paint bomb explode? ans: YESIf your answers to (d) and (h) differ, you need to explain the paradox, because either the engine room is covered with blue paint or not; you cannot have it both ways. If your answers are the same, you need to explain why.

    2. Relevant equations
    L=L0*sqrt(1-ß^2)

    3. The attempt at a solution
    I got part a thru g correct, what I could not understand is the answer to part (h) is yes. I believe from the tunnel's point of view the train (L=77.3m) has its rear passes thru the near end of tunnel (L0=273m) before its front passes thru the far end. This indicates that the bomb is deactivated first.
     
  2. jcsd
  3. May 19, 2016 #2

    SammyS

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    Well then, why did you say that the paint bomb explodes in this case?
     
  4. May 19, 2016 #3
    Because the correct answer is the bomb will go off
     
  5. May 19, 2016 #4

    TSny

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    I don't agree with this answer. Perhaps it's just a typographical error?
    Yes
    It's not that simple. Don't overlook this part of the problem:
    "However, when the rear car passes the near end of the tunnel (event RN), a device in that car is set to send a signal to the engine room to deactivate the bomb."
     
  6. May 19, 2016 #5

    SammyS

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    Oh! So those weren't your answers for a - h, they were the ' correct ' answers.

    What is the location of the bomb activation/deactivation system?
     
  7. May 20, 2016 #6
    Yes you're right about the contracted length, a typo it is indeed. The correct value should be 73.3m (3sig figs)

    Thanks a lot for highlighting the line where it says the "device sends a signal". I see that the signal being sent out does not assure that it reaches the front of the train in time.
     
  8. May 20, 2016 #7
    the bomb is located at the front of the train, while there is another device, located at the rear, which can send out a signal to deactivate the bomb. also I mistyped the answer to part a and e, the correct answer should be 73.3m
     
  9. May 20, 2016 #8

    SammyS

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    It's not clear as to what point the writer of the question intended to make. The entire train will be well out of the tunnel before the deactivation signal can reach the locomotive. This is true even if the deactivation signal is sent at the moment (in the train's frame) that the locomotive first enters the tunnel.

    Suppose we alter the problem in the following way, so that the relative delay in the deactivation signal is taken out of play.
    Let's place the blue paint bomb at the center of the train. A sensor at the front of the train sends a signal (perhaps a flash of light) to detonate the paint bomb as the front of the train passes the far end of the tunnel. Similarly, a sensor at the rear of the train sends a signal to deactivate the paint bomb as the the rear car passes the near end of the tunnel. ​

    Under this situation, the answers are the same as for the original situation (with the 73.3m corrected value). How do you explain answer h in this case?
     
    Last edited: May 20, 2016
  10. May 20, 2016 #9

    TSny

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    Can you show that it is impossible for the signal to reach the front of the train before the bomb goes off?
     
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