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!

Homework Help: Basic relativity.

  1. Feb 1, 2006 #1

    Some simple questions, I want to make sure I'm not making stupid mistakes before moving on. I have the questions and my answers, If you tell me that I got them incorrect, i'll run through my working or something (if there could be any..).

    1/ How fast must you travel towards a red light (650nm) for it to appear green (525nm)?

    Can this just be treated as length contraction?

    My Answer: 1.78x10^8m/s.

    2/ How great must the relative speed of two observers be for their time-interval measurements to differ by 1%?

    My answer: 4.2x10^7m/s.

    3/ A distant galaxy is moving away from the earth such that each wavelength is shifted by a factor of two; what is the speed of the galaxy relative to us?

    can this just be treated via length contracion?

    4/ A particle moves north at 0.8c, what is this particles speed as measured by an observed travelling south relative to the earth at 0.5c?

    5/Two spaceships of proper length 100m are travelling towards each other, each at a speed of 0.8c, measured on earth.
    (i) how long is each ship as measured by the terrestial observer?
    (ii)how long is earch ship when measured from the rest-frame of the other?

    Thats all the questions I have just now. My idea is that, since I haven't done relativity for ages I should try to brush up on this simple stuff before my new course starts. tomorrow i'm hitting the library for a textbook, sigh.
  2. jcsd
  3. Feb 1, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    No. This, and #3, are examples of the relativistic Doppler effect.
  4. Feb 1, 2006 #3
    I'm remembering something about the redshift parameter (z) being the change in wavelength over the 'rest'-wavelength?

    I think I've followed on from that:

    v/c = ((z+1)^2 - 1)/((z+1)^2 + 1)?

    is that what I'm looking for? thanks for the response by the way.

    (+sorry for the lack of `tex, I'm on the verge of not thinking straight enough to use it.)
  5. Feb 1, 2006 #4

    Doc Al

    User Avatar

    Staff: Mentor

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook