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Homework Help: Rotational Motion and Angular Displacement

  1. Nov 19, 2007 #1
    1. The problem statement, all variables and given/known data

    A stroboscope is a light that flashes on and off at a constant rate. It can be used to illuminate a rotating object, and if the flashing rate is adjusted properly, the object can be made to appear stationary.

    (a) What is the shortest time between flashes of light that will make a three-bladed propeller appear stationary when it is rotating with an angular speed of 12.6 rev/s?

    (b) What is the next shortest time?

    2. Relevant equations


    3. The attempt at a solution

    I took ∆θ = ⅓ rev (2pi radians/1 rev) = 2.09 radians
    then I used ω=(∆θ)/(∆t)...
    12.6 = (2.09/t) which gives me t=.166

    What I did makes sense to me, but the answer is wrong so obviously I am too...Don't know what else to do...
  2. jcsd
  3. Nov 19, 2007 #2
    Looks like you have an issue with unit consistency. You have put the angular displacement in radians, but the angular velocity is in rev/s. Change velocity to rad/s.
  4. Nov 19, 2007 #3
    i'm retarded....

  5. Nov 19, 2007 #4
    No, no...don't be so hard on yourself. I like to refer to these moments as "Brain Farts". Not quite as permanent!
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