Doppler effect with moving source and observer

Tags:
1. Mar 29, 2015

hopelessphysics

1. The problem statement, all variables and given/known data
As you drive at a speed of 15.0 m/s an ambulance approaches you from behind with its siren blaring at a frequency of 1022 Hz. After the ambulance has passed you the signal is now 981 Hz. If both you and the ambulance travel at a constant speed how fast was the ambulance traveling?

2. Relevant equations
fo=fs(1+- vo/v)/(1+-vs/v)

3. The attempt at a solution
So I tried to use the equation above by plugging in the frequencies, but was trapped because the frequencies show only what the source was emitting, and not what the observer was hearing.

Last edited by a moderator: Mar 30, 2015
2. Mar 29, 2015

Staff: Mentor

fo is the observer frequency.
You don't have the source frequency, but you have a second equation you can use.

3. Mar 29, 2015

hopelessphysics

What is the second equation?

4. Mar 29, 2015

Staff: Mentor

The ambulance before and after it passes you => 2 equations