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: Rotational Speed with the Speed of Light

  1. Mar 16, 2005 #1
    An early method of measuring the speed of light makes use of a rotating slotted wheel. A beam of light passes through a slot at the outside edge of the wheel, travels to a distant mirror, and returns to the wheel just in time to pass through the next slot in the wheel. One such slotted wheel has a radius of 11.0 cm and 800 slots at its edge. Measurements taken when the mirror was L = 500 m from the wheel indicated a speed of light of 3.0 x10^5 km/s.

    My work:
    L = 500m

    Arclength = 2pi*.11m
    Rotational speed = (2pi*.11m)/800
    Rotational speed = ((2pi*.11m)/800) * Speed of Light
    Rotational speed = 259181.39 rads/s <- This is wrong

    I did (Arclength/Slots) * Speed of Light to calculate 1 slot's worth.

    What was the (constant) rotational speed of the wheel? (rad/s)

    What was the translational speed of a point on the edge of the wheel? (m/s)
  2. jcsd
  3. Mar 16, 2005 #2
    You have not even used the length L.

    Time of travell for the light to travell from the disc and back is t= 2L/v where v is the given speed of light. Now within this time how much the disc has rotated? [tex]2\pi / 800[/tex].

    Angular speed is anlgle in rad / time = 2357 rad/s

    Tangantial speed is simply radius * angular speed.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook