1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Relativity question help, my brain has shut down

  1. Oct 23, 2012 #1
    ive been practising relativity questions and theres one i cant do, ive tried loads of ways of approaching it and i cant go any further hence asking for help

    it says " the diameter of the earths orbit is 3.0 x 10^8 m. a spaceship crosses the orbit in 750s, as measured in the spaceship. what is the speed of the spaceship relative to the earth?"

    any guidance would be much appreciated,
     
  2. jcsd
  3. Oct 23, 2012 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    welcome to pf!

    hi dave2001! welcome to pf! :smile:

    (try using the X2 button just above the Reply box :wink:)
    ok, so in the sun's frame x = vt = 3 108,

    and in the spaceship frame t' = 750,

    sooo … ? :smile:
     
  4. Oct 23, 2012 #3
    oh sorry , just seen it now you have said that (x2)
     
  5. Oct 23, 2012 #4
    Re: welcome to pf!

    sorry, thats how mashed my head is, its not 3 108 thats the value for c,

    the diameter of the orbit is given as 3.0 x 1011 m
     
  6. Oct 23, 2012 #5
    the diameter of the earths orbit is 3.0 x 1011 m. a spaceship crosses the orbit in 750s, as measured in the spaceship. what is the speed of the spaceship relative to the earth? (c = 3.0 x 108 ms-1
     
  7. Oct 23, 2012 #6
    Re: welcome to pf!

    i got that far but v comes out at 4x108 ms-1 and it doesnt work
     
  8. Oct 23, 2012 #7
    the answer is 2.4x108 ms-1

    cant figure out how to get to that
     
  9. Oct 23, 2012 #8

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    hi dave2001! :smile:

    you have been busy while i was out! :biggrin:
    show us how you got that …

    presumably you started by finding t = 1000 ?
     
  10. Oct 23, 2012 #9
    yeah that was the first assumption i made

    distance x = 3x1011 m
    t'=750
    t=1000

    so in frame for t, v=3x108
    for frame t' v=(3x1011)/750 gives 4x108ms-1

    i gotta feeling im going about this the totally wrong way
     
  11. Oct 23, 2012 #10
    im getting nowere either with the f-l contraction
     
  12. Oct 23, 2012 #11

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

  13. Oct 23, 2012 #12
    3x108 x 750 = 2.25x1011m
     
  14. Oct 23, 2012 #13

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    (it'll be a lot easier if you stick to writing c instead of 3x108 :wink:)

    yes, but i mean what is the formula with ct' on the LHS and x and t on the RHS? :smile:

    hint: in the sun's frame, t = x/v = 1000c/v, isn't it? :wink:
     
  15. Oct 23, 2012 #14
    you mean in the earths frame, yeah i get the time is 1000c/v,
    as for the equation i dont know, i cant find anything in the textbook, im sorry been working on this question since yesterday and its really getting to me, i keep getting stuck at this point :-(
     
  16. Oct 23, 2012 #15
    keep coming out with 1.98x108ms-1
     
  17. Oct 23, 2012 #16

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    no, in the sun's frame :wink:
    i keep telling you …

     
  18. Oct 23, 2012 #17
    why the suns frame?
     
  19. Oct 23, 2012 #18

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    it's "the diameter of the earth's orbit",

    which is stationary in the sun's frame,

    but obviously not in the earth's frame!! :smile:
     
  20. Oct 23, 2012 #19
    so ct'=γ(ct-βx) with β=v/c, and γ=1/√1-β2
     
  21. Oct 23, 2012 #20

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    ok, now write that out in terms of β and x (with no γ or t), and solve :smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook