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Homework Help: Finding Speed using Doppler Shift

  1. Oct 30, 2011 #1
    1. The problem statement, all variables and given/known data

    Light with a frequency of 6.1(1014) Hz is measured from a star known to produce light with a frequency of 6.0(1014). How fast is the star moving toward or away from earth?

    2. Relevant equations
    f = f0(v + vo)/(v-vs.

    f=observed frequency
    f0 = original frequency
    v=wave speed
    vo = wave of observer
    vs=wave of source

    3. The attempt at a solution

    6.1(1014) = 6.0(1014)(3.0x108 + 0)/(3.0x108 + x)

    -> (3.0x108 + x) = 6.0(1014)(3.0x108 + 0)/6.1(1014)

    -> x = ( 6/6.1 x (3.0(108) ) - 3.0x108

    -> -4918032.787 m/s
    Moving away at 4918032.787 m/s

    I feel that I'm doing it wrong since shouldn't it be moving toward earth since the frequency observed is higher?
  2. jcsd
  3. Oct 30, 2011 #2


    User Avatar

    Staff: Mentor

    Your equation was: f = f0(v + vo)/(v-vs)
    where vs is the speed towards the observer which gives that higher f.

    You changed it, for some reason, and used +vs
    which, to still be correct, means your vs is now the negative of its speed towards the observer.

    You calculated this to be -4918032.787 m/s,
    so take the negative of that and you have the speed towards the observer. :smile:

    I haven't checked your arithmetic.
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