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Lorentz transformations (2nd year relativity)

  1. Oct 11, 2010 #1
    1. The problem statement, all variables and given/known data

    A light signal is sent from the origin of a system K at t = 0 to the point x = 1 m, y = 8 m, z = 13 m. a) At what time t is the signal received?
    b) Find ( x', y', z', t' ) for the receipt of the signal in a frame K' that is moving along the x axis of K at a speed of 0.6c.

    2. Relevant equations

    The lorentz transformations:

    x' = [tex]\gamma * (x - vt)[/tex]
    y' = y
    z' = z


    3. The attempt at a solution

    Part a was easy. I got the right answer. I just took the length of the vector given by the co-ordinates and divided by the speed of light. The answer is [tex] 5 * 10^8 [/tex] I am having trouble with part b.

    Ofcourse, y' and z' were easy to get. t' (i had 3 tries, and i used them all) so I lost a mark there. I have one try left on x'.

    Using the equation, we first have to solve for [tex]\gamma[/tex]. Plugging the numbers into the equation for gamma:
    [tex] \frac{1}{\sqrt{1-(v/c)^2}}\; \text{yields} \; 1.25[/tex]

    Then using the lorentz transformation I have the following eqn:
    x' = [tex]\gamma[/tex] (x - vt) . Plugging in the numbers yeilds
    x' = 1.25(1 - 0.6 * c *(5E-8))

    I get 10 as the answer. It is wrong. I also thought t = 0 could work since thats when the event happened. But the answer 1.25 is also wrong.

    My third attempt yielded 11.25m however, I am scared to submit it. If anyone can please verify my number for me.
     
  2. jcsd
  3. Oct 11, 2010 #2

    diazona

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    Homework Helper

    The Lorentz transformations are more accurately written
    [tex]\Delta t' = \gamma(\Delta t - v\Delta x/c^2)[/tex]
    [tex]\Delta x' = \gamma(\Delta x - v\Delta t)[/tex]
    The [itex]\Delta[/itex] indicates that the numbers to be plugged in should be the difference between two spacetime events. So putting in t=0 is a mistake that you should not make again.

    Can you show your calculations for the other two attempts?
     
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