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: Simple relativity problem

  1. Jan 4, 2013 #1
    1. The problem statement, all variables and given/known data

    An observer fires a laser pulse at a stationary mirror located at distance d away and measures the time taken. A second observer moves in the same direction as the beam with velocity v and sees these events.

    According to each observer, how much time will it take for the pulse to make each leg of the trip and back and what is the total time taken from emission to detection?

    3. The attempt at a solution

    Event 1: Laser pulse is fired

    t1 = 0, x1 = 0

    t1' = γ(t1 - vx1/c2) = 0

    Event 2: Laser pulse bounces off mirror

    t2 = d/c

    = γ(t2 - vx2/c2)
    = γ(d/c - vd/c2)
    = (γd/c)(1 - v/c)

    Event 3: Laser reaches back observer

    t3 = 2d/c, x3 = 0

    t3' = γ(t3 - vx3/c2)
    = γ(2d/c)
    = (2d)/√(c2 - v2)

    Is there anything wrong with my working?? I can get to the first to the last step by simply applying the time dilation effect by using Δt' = γΔt...
  2. jcsd
  3. Jan 4, 2013 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Everything looks good. And yes, the first to last step can also be obtained using the time dilation formula. Good.
  4. Jan 4, 2013 #3
    Thanks! That was pretty easy for a 10 mark exam question o_O
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook