1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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)


    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!!


  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
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Doppler Effect help!