Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Example and question

  1. Sep 11, 2009 #1
    Hi guys
    is the example true?
    what will the clock 3 count when the light reach it

    Last edited: Sep 11, 2009
  2. jcsd
  3. Sep 11, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF! Move towards the light!

    Hi ghost11! Welcome to PF! :smile:

    Yes, the diagrams are correct …

    the lamp and the clocks are stationary, and the plane is going half as fast as the light, so in the clocks' frame (middle diagram), the plane will reach the first clock at the same time as the light reaches the second clock.

    And in the plane's frame (bottom diagram), a ray of light is moving left, and the clocks are moving right, half as fast as the light …

    (and of course they're closer together, but that won't make any difference)

    so when the first clock reaches the plane, the third clock will have moved towards the light.
    You do the maths (no square-roots needed! :wink:) …

    what equations do you get?

    Hint: move towards the light, ghost11! :smile:
  4. Sep 11, 2009 #3
    thank you tiny-tim
    in the second and third diagram the clocks count 1 second!!
    in the second diagram it will take 1.5 second for the light to reach clock 3
    in the third diagram it will take 1 second for the light to reach clock 3
    what the clock will count for real??
    please explane more ... cause iam little slow .. o:)
  5. Sep 11, 2009 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Clock 3 will show 1.5s when the light reaches it. The third diagram is a bit confusing. It shouldn't say "clocks 1s". It should say that the clock on the plane shows 1s. Also, it shouldn't say that the plane's speed is c/2. It should say that the speed of the light bulb and the clocks in the picture is c/2.
  6. Sep 11, 2009 #5
    Fredrik thank you a lot
    I didn't see it that way ... i understand now
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook