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Homework Help: Physics Question Involving Angular Speed and Angular Velocity.

  1. Nov 27, 2011 #1
    1. The problem statement, all variables and given/known data
    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, as in Fig. 10-29, 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 5.6 cm and 75 slots at its edge. Measurements taken when the mirror is L = 800 m from the wheel indicate a speed of light of 3.0 x 105 km/s. (a) What is the (constant) angular speed of the wheel? (b) What is the linear speed of a point on the edge of the wheel?


    2. Relevant equations


    angular velcity= answer is part a *.056m

    3. The attempt at a solution

    For part (a)
    I did use =angle/time

    angle I solve (2Pi)/75 = .083776

    I used t = 2L/c = 2(800)/(3x10^8) = .000005 sec

    Then divided .083776/.000005 = 16755.2 rad/s

    For part (b)

    = my answer is part(a) * .056m

    =938.291 rad/s

    I know I'm doing something wrong, but don't know what it is. Can anyone help?
  2. jcsd
  3. Nov 27, 2011 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    I get 15,707 rad/s. You're introducing a huge error by prematurely rounding the value of the Δt to 5e-6 s when it should be 5.333333333333e-6 s. Keep all significant figures in intermediate calculation steps. Round only the final answer at the end.
  4. Nov 27, 2011 #3
    You are right I rounded to early so it affected by final answer thank you very much and I hope you had a Happy Thanksgiving as well.
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