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: PLEASE HELP: Relativity and Relativistic Kinematics

  1. Mar 18, 2009 #1
    Here is the question that i am struggling with.
    A motorist passes a red traffic-light (wavelength = 650 nm). The motorist’s velocity
    makes an angle theta to the direction of light from the traffic light. Write down in four-
    vector and matrix form the relationship between the four-momentum of a photon as
    seen in the motorists’ rest frame, and as seen in the rest frame of the traffic light.

    Using the above result or otherwise, assume that the motorist was travelling directly
    towards the traffic-light, and is stopped by the police. The motorist tells the police-
    officer that he observed the traffic-light as being green (wavelength = 500 nm). What
    was the motorist’s speed as a fraction of the speed of light?

    All i know is that the 4-vector for a photon is the same (px,py,pz,E/c) and that as the photon is massless, the magnitude is 0. However i am unsure if i am meant to be incorporating doppler shift into this ?

    all help will be much appreciated!
  2. jcsd
  3. Mar 21, 2009 #2
    Look up the Doppler Shift
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook