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Homework Help: Angle of a rod (relativity)

  1. Feb 14, 2009 #1
    I don't know how to edit the title but the problem is solved.

    One needs two simultaneous events in frame S not in S'.
    (Just in case someone tried to figure it out)

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


    this is not a homework, but an exercise of the book Introduction to Special Relativity by Wolfgang Rindler
    (Oxford Science Publications):

    In S' a straight rod parallel to the x'-axis moves in the y'-direction with constant velocity u.
    Show that in S the rod is inclined to the x-axis at an angle


    [tex]\theta=-arctan( \frac{\gamma u v }{c^2})[/tex]

    2. Relevant equations


    [tex] (1)~ x=\gamma (x' + v t')[/tex]
    [tex] (2)~ t = \gamma(t' + \frac{v x'}{c^2})[/tex]

    3. The attempt at a solution

    The x and x' axis of S and S' are parallel. The beginning point of the rod may be placed
    in x'=y'=0 at t'=0.
    For t'=0 I get two events in S for the beginning and end point of the rod, which are [with (1) and (2)]

    [tex]P_1 : x=0, t=0[/tex]

    [tex]P_2 : x=\gamma L, t=\gamma \frac{v L}{c^2}[/tex]

    And u_y' = u transforms like

    [tex]\frac{u}{\gamma (1 + \frac{v u_x'}{c^2})}[/tex]

    with u_x' = 0 this is

    [tex]u_y = \frac{u}{\gamma} [/tex]

    so with

    [tex]\Delta y = u_y t = \frac{u v L}{c^2}[/tex]

    [tex]\tan(\theta) = \frac{\Delta y}{\Delta x} = \frac{u v}{\gamma c^2} [/tex]

    which is not the same as the solution given above.

    Last edited: Feb 14, 2009
  2. jcsd
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