# Homework Help: Doppler Effect 2

1. May 29, 2010

### roam

1. The problem statement, all variables and given/known data

An ambulance is traveling at 84.0 km/h. The ambulance has a siren which produces sound at a frequency of 1.7 kHz. Assume the speed of sound in air is 345.0 m/s.

A police car is traveling at a constant speed of 80.0 km/h directly towards the ambulance. The police car has a siren which produces sound at a frequency 3.2 kHz. The sound from the siren reflects off the ambulance. If the time taken for the sound of the siren to travel from the police car to the ambulance and back again is 0.15 s, how far is the police car from the ambulance at this moment? (You can neglect the distance the police car has moved in this time)

2. Relevant equations

General Doppler-shift expression: $$f'=\left( \frac{v+v_o}{v-v_s}\right) f$$

3. The attempt at a solution

The answer is 25.9 meters, but I've absolutely no idea what strategy to use in this problem. Any guidance is greatly appreciated.

2. May 30, 2010

### irycio

Lol :D That doesn't have to do anything with a Doppler effect. Well, the answer given it seems to be a very simplified exercise-just calculate the distance that a sound wave makes in 0.15 s and divide it by 2 (there and back againg .