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Roaku

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**Doppler Shift/ Doppler effect! HELP**

**1. A police car is traveling at a constant speed of 110. km/h with a radar gun which generates and detects radiowaves. The police car and the ambulance drive directly towards each other. The radar gun produces radiowaves with a wavelength of 0.0440 m.**

An ambulance is traveling at 64.0 km/h towards an intersection

The ambulance has a siren which produces sound at a frequency of 4.30 kHz. Assume the speed of sound in air is 340. m/s.

(i) What is the relative speed between the police car and the ambulance?

(ii) What doppler shift (Δf = f′–f) is detected by the radar gun after the radiowave has been reflected by the ambulance?

An ambulance is traveling at 64.0 km/h towards an intersection

The ambulance has a siren which produces sound at a frequency of 4.30 kHz. Assume the speed of sound in air is 340. m/s.

(i) What is the relative speed between the police car and the ambulance?

(ii) What doppler shift (Δf = f′–f) is detected by the radar gun after the radiowave has been reflected by the ambulance?

## Homework Equations

relative speed = v1 + v2

f obs = fs ((vw + v obs) / (vw + vs))

Vw = 340 m/s (speed of sound...)

v obs = 64km/h (17.778 m/s)

vs = 110 km/h (30.556 m/s)

fs = 4.3 kHz

(Δf = f′–f)

f = 4.3 kHz

v = fλ (I got λ = 0. 044 so what am I using it for??)

## The Attempt at a Solution

A) Easy 110 +64 = 174 km/h

B) The problem child.

I plug in with the equation above, with + because the ambulance and the police are moving towards each other, but I don't get the right answer (2,200 Hz)

I know I need to find Δf, but how do I go about to find it? Why is this equation not working, and which equation am I meant to use? Whats the radar wavelength for? How do I implement it?

Any help would be much appreciated!