# Doppler Effect: Ambulance

1. Dec 6, 2011

### Lizziecupcake

So I'm having a hard time getting the second part of the problem, so could anyone help me

An ambulance travels down a highway at a speed of 65.0 mi/h, its siren emitting sound at a frequency of 4.10 102 Hz. Take the speed of sound in air to be v = 345 m/s. What frequency is heard by a passenger in a car traveling at 56.0 mi/h in the opposite direction as the car and ambulance

a)approach each other: 480.2 Hz
b)pass and move away from each other?: 370.7 Hz

Repeat this problem, but assume the ambulance and the car are going in the same direction, with the ambulance initially behind the car. The speeds and frequency of the siren are the same as in the example.
(a) Find the frequency heard before the ambulance passes the car.
(b) Find the frequency heard after the ambulance passes the car. [Note: The highway patrol subsequently gives the driver of the car a ticket for not pulling over for an emergency vehicle!]

So far I tried to do the same as the first part by changing the speed because I'm not exactly sure what to do.

The equation used is:
fO= fS(v + vO)/(v - vS)

2. Dec 6, 2011

### PeterO

No doubt you did the first part using a closing speed of 121 mi/h and a separating speed of 121 mi/h [ie 65 + 56]

With the vehicles travelleing in the same direction, the closing and opening speeds are just 9 mi/h [ie 65 - 56]

3. Dec 6, 2011

### Lizziecupcake

Can you clarify a bit more, I'm still confused. Also, I converted the speeds into m/s

4. Dec 6, 2011