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: Doppler effect ambulance problem

  1. Aug 5, 2006 #1
    Question: An ambulance with a siren emitting a whine at 1300 Hz overtakes and passes a cyclist pedaling a bike at 2.44 m/s. After being passed, the cyclist hears a frequency of 1280 Hz. Use 343 m/s for the speed of sound in air. How fast is the ambulance moving?

    i set up the doppler equation with as follows:

    1300 hz =1280 hz((343+(2.44/343))/(343+(vs/343))), I am pretty confidant the signs I choose are correct but must be making a mistake in either interpreting the data or algebra, any help is greatly appreciated..
  2. jcsd
  3. Aug 5, 2006 #2
    If you are not sure about the positive direction, I can tell you that the positive direction is aimed towards eachother relative to the air.
  4. Aug 6, 2006 #3
    when both the source and the cyclist move (in this case), then the Dopplereffect is given by:

    [tex]f_{cyclist} = f_{ambulance}\frac {(v + v_{cyclist})} {(v + v_{ambulance})}[/tex]

    but you can also say that the cyclist stands still and the ambulance passes with a relative velocity [tex]v_{rel}[/tex]. Then use:

    [tex]f_{cyclist} = f_{ambulance}\frac {v} {(v + v_{rel,amb})}[/tex] and then add the velocity of the cyclist.
    Last edited: Aug 6, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook