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Homework Help: Doppler Effect help!

  1. Mar 13, 2007 #1
    Hey guys...or ladies....um....just wondering what I hav done wrong for the following Q, it is an online Q i am doing and it tells me my answer is incorrect???


    Q) "A car alarm is emitting sound waves of frequency 520 Hz. You are on a motorcycle, traveling directly away from the car."

    "How fast must you be traveling if you detect a frequency of 490 Hz?
    Enter your speed as a positive number. Take the speed of sound to be 344 m/s.


    It seems fairly striaght forward, so I used the doppler effect (ie the wavelength must be greater and hence the frequency less if moving away)

    so...

    f(effetive) = v/(v+vl) . f

    where vl = velocity of listener...

    so i have

    344/(344 . vl) . 520 = 490

    solving for vl gives....21.06 ms^-1 roughly

    so therfore, bke must be moving at 21.06ms^-1 ?

    BUT it tells me this answer is incorrect??

    any ideas appreciated!!

    cheers

    rhys
     
  2. jcsd
  3. Mar 13, 2007 #2
    Nothing is wrong here. You may have to check if the car is stopdead or it may be moving at a certain speed.
     
  4. Mar 13, 2007 #3
    mmm....program must b dodgi....cheers
     
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