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H.W. Question about Relativity

  1. Jul 29, 2015 #1
    1. The problem statement, all variables and given/known data
    Rocket "A" velocity is 0.8c to the right ,reltaive to earth
    Rocket "B" velocity is 0.6c to the left , relative to rocket "A"
    (This one I succedeed)

    My problem was in this question:

    Repeat the same problem ,but bow rocket "A" moves 0.8c * upwards *(positive y axis)
    For some reason , I didn't make well


    2. Relevant equations

    ec64af893d22a57ae43315ae30f82449.png

    3. The attempt at a solution

    Tried using these equation while:

    V=0.8,vx'=-0.6c,vy'=-0.8

    Now , I know I got something wrong but I couldn't figure it out by myself(although I tried).....

    Thank you!
     
    Last edited by a moderator: Jul 29, 2015
  2. jcsd
  3. Jul 29, 2015 #2

    TSny

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    Hello.

    Please state what the question is asking for.
     
  4. Jul 29, 2015 #3
    Oh sorry ,forgot the th most important part
    The question is :

    "What is the velocity of Rocket B , relative to earth ?"
     
  5. Jul 29, 2015 #4

    TSny

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    OK

    Just to be totally clear, what frame are you taking to be the "unprimed" frame? Is it the earth frame, the A-rocket frame, or the B-rocket frame?
    Same question for the "primed frame".
     
  6. Jul 29, 2015 #5
    Ok ,let's use the following notations

    Earth- S
    Rocket A- S'
    Rocket B- S''(not necessary)
     
  7. Jul 29, 2015 #6

    TSny

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    For your list of relevant equations, what basic assumption is made about the direction of the velocity of the primed frame relative to the unprimed frame?
     
  8. Jul 29, 2015 #7
    let's say we define the "right" to be the x positive direction and assume rocket A (O') moves in this direction realtive to O (originally S ,S' but it doesn't matter)

    1280px-Frames_of_reference_in_relative_motion.svg.png
     
  9. Jul 29, 2015 #8
    Oh sorry again ,wrong diagram
    S' goes in the y positive direction(upwards) relative to S
     
  10. Jul 29, 2015 #9

    TSny

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    OK. Back to my earlier question. The equations that you wrote down as the Relevant equations are only valid if the primed frame moves in what direction relative to the unprimed frame?
     
  11. Jul 29, 2015 #10
    Sorry , I misunderstood the question
    However ,I will try answering it: I guess these equations are true If S' goes to the right relative to S
    (So ,does it mean ,they can't fit the directions I just presentd ?)
     
  12. Jul 29, 2015 #11

    TSny

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    The formulas you listed assume that the primed frame is moving in the x direction relative to the unprimed frame.
    Since rocket A (frame S') moves in the y direction relative to the earth (frame S), you will not be able to use the equations as written.

    Can you see how to modify the equations so that they are applicable?
     
  13. Jul 29, 2015 #12
    Maybe ....
    in the eqaution of vx should I use Vx
    -----------------v
    x-------------------Vy
    -----------------v
    z------------------Vz

    ?
     
  14. Jul 29, 2015 #13

    TSny

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    I'm not sure I'm following you here.

    In the equations that you listed, the relative motion of the two frames is assumed to be along the x direction. In your problem the relative motion is along the y direction. So, you can just replace all the x and x' subscripts in the equations by y and y' subscripts. Likewise, replace all y and y' subscripts in the equations by x and x' subscripts.
     
  15. Jul 29, 2015 #14
    Ohhh you are right

    Thank you very much for your patience and for your help!
     
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