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Homework Help: Relativity Problem

  1. Apr 20, 2008 #1


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    1. The problem statement, all variables and given/known data

    A ferry boat headed directly away from the river bank is traveling at 3 km/h with respect to the water in a river flowing at 4 km/h. A passenger walks diagonally toward the stern of the boat with a speed of 1.39 m/s. The passenger's path is at an angle of 53.1 degrees toward the upstream direction from the length of the boat. What is the passenger's speed relative to the bank?

    2. Relevant equations


    3. The attempt at a solution

    I know the speeds form a triangle with leg lengths 3 and 4 and hypotenuse 5. I'm not sure where to go from there, though....
  2. jcsd
  3. Apr 20, 2008 #2
    It looks like you might have to use 2 lorentz transformations. First, i would set up my rest frame, S, to be the water, and the moving S' frame to be the boat. I would then find the velocity of the person in the reference frame of the water, S. I would then use the lorentz transformations again, setting my new rest frame to be the bank and my moving frame to be the water. This way you willk now the velocity of the person relative of the water and will be able to transform it to the bank. this might be wrong though -- my spec rel is not great.
  4. Apr 20, 2008 #3


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    Hi bfr! :smile:

    erm … are you sure this is a relativity question? :confused:

    The speeds are far too small for relativity to be relevant … I think you're expected to treat it as a straightforward vector addition problem. :smile:
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