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Homework Help: What beat frequency will the observer detect?

  1. Mar 13, 2008 #1
    1. The problem statement, all variables and given/known data
    Two trains emit whistles of the same frequency, 282 Hz. If one train is at rest and the other is traveling at 45 km/h away from an observer at rest, what beat frequency will the observer detect?

    2. Relevant equations

    3. The attempt at a solution

    I used listener behind formula and got an answer of 544 Hz because i doubled the Frequency source. The answer is wrong, please help me!Thanks!
  2. jcsd
  3. Mar 13, 2008 #2
    Why don't you show your working out.
  4. Mar 13, 2008 #3
    While i was waiting i looked over it again and this is what i did for another answer i got

    F(listener behind)=F(of source) [v/v+v(source)]

    I did one for each since there are two trains.
    For the train on rest, i did F(listner behind)=282*[343/(343+0)] which is 282 Hz

    For the train going 12.5 m/s i did, F(lb)=282*[343/(343+12.5)] and got 272 Hz

    So then i subtracted 282-272 and got 10 Hz, is this the answer?
  5. Mar 13, 2008 #4
    That's the one.
  6. Mar 13, 2008 #5
    Thanks! I'm also having major trouble with an intensity problem, can you help me with that? Thanks!
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