1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Calculate the frequency of the siren

  1. Apr 13, 2008 #1
    1. The problem statement, all variables and given/known data

    A police car is moving towards the east at 34.0 m/s while sounding its siren. There is a wind blowing from the west with a speed of 15.0 m/s. A second police car is ahead of the first police car and is sounding its identical siren while moving towards the east at 32.0 m/s. A policeman in the first police car hears a beat frequency of 4.80 Hz between the two sirens. The speed of sound in air is 343. m/s.
    (a) the frequency of the siren;
    (b) the beat frequency heard by the policeman in the second police car;
    (c) the frequency from the siren of the faster police car heard by a person standing on the sidewalk between the two cars.

    (a) 864.0 Hz
    (b) 5.333 Hz
    (c) 954.7 Hz

    2. Relevant equations


    3. The attempt at a solution

    i think v=343-15
    Besides the above, I'm Clueless. Can someone help me out? (no solutions please)
    Thanks guys
  2. jcsd
  3. Apr 14, 2008 #2

    my first step would be the transformation into a reference frame that is moving with the same velocity as the wind. thereby you take care of the wind once and dont have to consider it during the further calculations. so the transformed velocities of the objects are:

    first police car: 19 m/s
    second police car: 17 m/s
    person: -15 m/s
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook