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Homework Help: Two trains and a tree

  1. Jan 9, 2016 #1
    1. The problem statement, all variables and given/known data
    Two trains of proper length L move toward each other in opposite directions on parallel tracks. They both move at speed v with respect to the ground. Both trains have clocks at the front and back, and these clocks are synchronized as usual in the frame of the train they are in. A tree is located on the ground at the place where the fronts of the trains both read zero when they pass. Find the reading on the clocks at the backs of the train when they (the backs) pass each other at the tree. Do this in three different ways:
    a) Standing next to the tree on the ground, and you observe what one of the rear clocks is doing.
    b) Imagine that you are on one of the trains, observing what your own rear clock is doing during the time the tree travels relevant distance.
    c)Imagine that you are one of the trains, observing what the other train's rear clock is doing during the time the tree travels the relevant distance.

    2. Relevant equations
    I'm not even sure what equations are even relevant for this problem.

    3. The attempt at a solution
    I am so lost and don't even know where to begin with this problem. Special relativity just isn't my thing. Can someone please at least help in pointing me in the right direction?
  2. jcsd
  3. Jan 9, 2016 #2
    I'm not quite sure about c but I believe a and b are simple. If it is asking what I think it is asking, use the time dilation equation for using the proper time interval to find the dilated time interval. Just remember that the tree clock has the proper time interval. Then, if you need to explain it in words, remember that a larger time interval means there is more time between ticks on a clock.
  4. Jan 10, 2016 #3


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    It's difficult for us to help you with your homework if you are so lost with the material. My suggestion is you have to go back to the material and try again to understand it. This question is not the easiest, so I also suggest you look for some easier questions to get you going then come back to this one.

    Try revising:

    Time dilation
    Length contraction
    Relativity of simultaneity ("Leading clocks lag")

    And try some questions for each of these topics first.
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