1. Limited time only! Sign up for a free 30min personal 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!

Special Relativity rocket’s speed

  1. Jan 14, 2015 #1
    1. The problem statement, all variables and given/known data
    A rocket flies between two planets that are one light-year apart. What
    should the rocket’s speed be so that the time elapsed on the captain’s watch
    is one year?

    2. Relevant equations
    I have v = d'/t'
    d'=d/ϒ
    t'=1 year
    d=1 light year

    3. The attempt at a solution
    Will the equations I use vϒ=d/t' but I get complex roots as an answer. What am I doing wrong?
     
  2. jcsd
  3. Jan 14, 2015 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Hello and welcome to PF!

    I believe you have it set up correctly. You'll need to show your algebra steps in order for us to see where you are making a mistake.
     
  4. Jan 14, 2015 #3
    v(sqrt(1-(v/c)^2)=d/t
    v^2(1-(v/c)^2)=d^2/t^2
    v^2-v^4/c^2=d^2/t^2
    v^2-v^4/c^2-d^2/t^2=0
    I used wolfram to solve for roots and it gave back complex numbers.
     
  5. Jan 14, 2015 #4

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Did you use the correct expression for ϒ?
     
  6. Jan 14, 2015 #5
    oops, v^2/(1-(v/c)^2)=d^2/t^2
    Should I multiply the LHS by the denominators conjugate?
     
  7. Jan 14, 2015 #6

    TSny

    User Avatar
    Homework Helper
    Gold Member

    EDIT: OK

    No, try multiplying both sides by the denominator on the left side.
     
  8. Jan 14, 2015 #7
    Ah! thank you, everything works now.
    edit: is there any way I can give your points or something of that regard?
     
  9. Jan 14, 2015 #8

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Good work! Don't worry about any points.
     
  10. Jan 16, 2015 #9
    Hey! Would you mind sharing what you got as a result? I'm having a little trouble with the equation myself.
     
  11. Jan 16, 2015 #10

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    PF custom is to help find answers. So show your work to let us help you where you get stuck.

    @br0shizzle1: TSny is too modest -- befitting someone of his (/her?) status in PF. But there is a "Like" link at the lower right in every posting (except your own :) ).
     
    Last edited: Jan 16, 2015
  12. Jan 16, 2015 #11
    Fair enough. I also had v(gamma)=d/t' --> (v^2)=((d^2)/(t'^2))*(1-(v/c)^2) --> here I considered d^2/t'^2 to be equal to c^2 because d=1 light year and t we set to 1 year so my equation became --> v^2 = c^2(1-(v^2/c^2)) --> (v^2)/(c^2) = 1-(v^2/c^2) --> ((v^2)/(c^2))+((v^2)/(c^2)) = 1 --> 2((v^2)/(c^2)) = 1 --> ((v^2)/(c^2)) = 1/2 ----> v/c=sqrt(1/2) and then v=sqrt(1/2)*c.

    Appreciate the response :)
     
  13. Jan 16, 2015 #12

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    And where do you think you have trouble with the equation ?

    Oh, and: Hello Harjo, welcome to PF! :)

    A little unusual to make your debut tagging onto an existing thread, but your question is clear and it's a start...
     
  14. Jan 16, 2015 #13
    Oh I was just wondering if I had done it right. If my assumption was fair to make.

    I realize now I should have put the work down first and then asked for clarification. woopsie daisy
     
  15. Jan 16, 2015 #14

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You're doing fine. And, just so you know: I was surprised by the answer, too ! Shows you're never too old to learn
     
  16. Jan 16, 2015 #15
    Awesome! True enough :w

    Thanks a bunch!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Special Relativity rocket’s speed
Loading...