Doppler Effect ambulance speed Question

[Sheen91]

Homework Statement

An ambulance with a siren emitting a whine at 1530 Hz overtakes and passes a cyclist pedaling a bike at 2.22 m/s. After being passed, the cyclist hears a frequency of 1519 Hz. How fast is the ambulance moving? (Take the speed of sound in air to be 343 m/s.)

Homework Equations

http://img15.imageshack.us/img15/3340/equationx.jpg

The Attempt at a Solution

Since both the cyclist and the ambulance is moving it is slightly confusing for me.

I have tried this question multiple times but am unable to get the right answer.

I tried to the question where the ambulance is moving and the cyclist is stationary and then just add the velocity of the cyclist onto the velocity I get for the ambulance.

This is what i tried :

http://img18.imageshack.us/img18/426/answer.jpg

It is wrong but if someone is able to tell me what I did wrong that would be great.

Cheers

 

[Delphi51]

There is an error in the formula you are using. According to my text
it should be f' = f*v/(u + v)
where v is the speed of sound and u the speed at which the source is moving away from the observer. It comes out to 2.48, to which the 2.2 must be added.

 

[Sheen91]

So is the speed of the ambulance would be 2.48 + 2.2 = 4.68 m/s ??

Nice its right :D

Thanks