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Doppler Effect - Question

  1. Mar 7, 2009 #1
    1. The problem statement, all variables and given/known data

    An ambulance with a siren emitting a whine at 1530 Hz overtakes and passes a cyclist pedaling a bike at 2.22 m/s. After being passed, the cyclist hears a frequency of 1519 Hz. How fast is the ambulance moving? (Take the speed of sound in air to be 343 m/s.)

    2. Relevant equations

    http://img15.imageshack.us/img15/3340/equationx.jpg [Broken]

    3. The attempt at a solution

    Since both the cyclist and the ambulance is moving it is slightly confusing for me.

    I have tried this question multiple times but am unable to get the right answer.

    I tried to the question where the ambulance is moving and the cyclist is stationary and then just add the velocity of the cyclist onto the velocity I get for the ambulance.

    This is what i tried :

    http://img18.imageshack.us/img18/426/answer.jpg [Broken]

    It is wrong but if someone is able to tell me what I did wrong that would be great.

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 7, 2009 #2


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    Homework Helper

    There is an error in the formula you are using. According to my text
    it should be f' = f*v/(u + v)
    where v is the speed of sound and u the speed at which the source is moving away from the observer. It comes out to 2.48, to which the 2.2 must be added.
  4. Mar 7, 2009 #3
    So is the speed of the ambulance would be 2.48 + 2.2 = 4.68 m/s ??

    Nice its right :D

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