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Homework Help: Special Relativity with Three reference frames

  1. Jan 28, 2013 #1
    1. The problem statement, all variables and given/known data
    In the laboratory frame, event 1 occurs at x = 0 light-years, t = 0 years. Event 2 occurs at x = 6 light-years, t = 10 years. In all rocket frames, event 1 also occurs at the position 0 light-years and the time 0 years. The y- and z- coordinates of both events are zero in all frames.
    a) In rocket frame A, event 2 occurs at time t’ = 14 years. At what position x’ will event 2
    occur in this frame?
    b) In rocket frame B, event 2 occurs at position x’’ = 5 light-years. At what time t’’ will
    event 2 occur in this frame?
    c) How fast must rocket frame C move if events 1 and 2 occur at the same place in this
    rocket frame?

    2. Relevant equations
    s^2 = c^2Δt^2-Δx^2
    t = (1/√1-v^2/c^2)t'

    3. The attempt at a solution
    For part A, I did c^2Δt^2-Δx^2 = c^2Δt'^2-Δx'^2. Because the everything in the rocket frames start at 0, then the entire left side becomes 0 and I get. √c^2 * (14)^2 = Δx' which is 4.2*10^9 or 14c.
    I did the same thing for part B. The c^2Δt^2-Δx^2 = c^2Δt''^2-Δx''^2 and ended up with √5/c = 1.29*10^-4 = Δt''.
    For part c, I know that I need to use t = (1/√1-v^2/c^2)t' . However, if I set t=0, then I get v rocket = c ---which shouldn't be right. It should be some fraction of c. My question is if if I am looking at a third reference frame, then do I use the values for time and position from the one before it? So to find the v in rocket frame C, I need to only use the data from rocket frame B?
  2. jcsd
  3. Jan 28, 2013 #2


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    Welcome to PF!

    Hi scienceLilly75! Welcome to PF! :smile:

    (try using the X2 button just above the Reply box :wink:)
    No …

    i] if x is in light-years, and t is in years, then c = … ? :wink:

    ii] the separation (c2t2 - x2) has to be between events 1 and 2, so it isn't 0 ! :smile:
  4. Jan 28, 2013 #3
    But at the beginning the time and position of the event is 0, so the left side has to be zero right?

    Or do I need to find spacial separation between A and B, then spacial separation between B and C?
  5. Jan 29, 2013 #4


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    Hi scienceLilly75! :smile:

    (just got up :zzz:)
    sorry, but that doesn't even make sense …

    A B and C are frames, and there's no separation between frames (spatial or otherwise)

    separation is between two events

    so which two events are you going to use? :smile:

    (oh, and c = … ? :wink:)
  6. Jan 29, 2013 #5
    I am confused. I understand that there is no separation between frames but between events in frames. In that case, I am stuck. The only thing I know about the frame C is that event one occurs at t=0, x=0. If the events occur on the same place on the rocket, then x' also equals 0. But then that will leave me with t' = 0 using the spacial relativity equation. if t'=0, then it is not moving. Where am I going wrong?
  7. Jan 29, 2013 #6


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    Hi scienceLilly75! :smile:
    Why do you say "in frames"? Separation is between two events, period. :wink:

    Use event 1 and event 2. :smile:
  8. Jan 29, 2013 #7
    So to find the speed, I have to use info from event 1 and 2 how they appear in frame c? then I can use v = x/t?
  9. Jan 29, 2013 #8
    Make up a table. Label the rows Laboratory, Rocket A, Rocket B, Rocket C. Label the columns Event 1 Time, Event 1 Distance, Event 2 Time, Event 2 distance. Fill in the table with any information you have available. If there is information missing, fill in any algebraic variable name that you choose. Add a 5th column to the table in which you calculate the following quantity

    ((Event 2 Time) - (Event 1 Time))2 - ((Event 2 Distance) - (Event 1 Distance))2. This quantity must be the same for all rows of the table.
  10. Jan 29, 2013 #9


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    why do you need for speed for part (a) ? :confused:

    what is the separation between events 1 and 2 ?​
  11. Jan 29, 2013 #10
    Question 3 is a little ambiguous. I guess it is asking how fast rocket C is traveling relative to the laboratory frame of reference. The key to answering this question is recognizing that rocket C is physically present at both events.
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