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Calculating frequencies and velocity

  1. Aug 8, 2014 #1
    [Mentor's note - This thread was moved here from a non-homework forum, therefore it does not use the template.]

    Hi guys

    Need some help answering questions about frequencies and calculating velocity.
    I'm given two frequencies: 470Hz for an approaching object, 440Hz for when the object is moving away. Velocity of the speed of sound is 343m/s. Work out the frequency of the sound at the object. The receiver is stationary. What formula do I use to work this out?

    And once that's been calculated, can velocity be calculated using this formula? fr = fs(v/(v – vs)) where fr is frequency heard by the receiver, fs is frequency from the sender, v is velocity of sound through air, and vs is velocity of the object.
    Any help is appreciated.
     
    Last edited by a moderator: Aug 10, 2014
  2. jcsd
  3. Aug 8, 2014 #2

    billy_joule

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    I think this belongs in the homework and coursework part of the forum, but anyway..

    The first equation here:

    http://en.wikipedia.org/wiki/Doppler_effect

    9150f5015026a8e96d0e7d6bbe11bb24.png

    plug in you data for the two cases (coming and going), you'll have a pair of linear equations, solve them for the two unknowns (Velocity of source and emitted frequency).


    There is a much faster, intuitive way of getting emitted frequency ( and then Vs) which will hopefully become obvious when you see it's value after calculating it the longer way...
     
  4. Aug 9, 2014 #3
    I've got an answer for my first question. Is it supposed to be 455Hz?
     
  5. Aug 9, 2014 #4
    catfish, I think we also need to know if the speed of the object when it's moving away is equal or not to the speed when the object is approaching.
     
  6. Aug 9, 2014 #5
    I think it's assumed that it is but I apparently also have to work out the speed of the object using the information and the formula provided.
     
  7. Aug 9, 2014 #6

    billy_joule

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    Science Advisor

    Yes, it's just the average of the coming and going frequency: (470+440)/2

    Now just plug some numbers in and solve for Vs
     
  8. Aug 9, 2014 #7
    Sweet, thanks for the help.
     
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