- 2

- 0

**1. Homework Statement**

A listener L hears the siren from a moving police car ,S, which is moving away from the listener with a speed of v = 20.0 m/s. The listener is moving at 10.0 m/s towards the car. There is a wind blowing with a speed of v(wind)= 10.0 m/s from S to L. The siren is emitting sound at a frequency (f) of 800 Hz. The velocity of sound in air is 343 m/s. Calculate the frequency (f') heard by the listener and the wavelength (λ) of that wave.

**2. Homework Equations**

f' = f(v +/- vD) / (v -/+ vS)

v = fλ

**3. The Attempt at a Solution**

Finding the new frequency is easy:

f' = (800hz)(343+10+ 10)/(343+10+20)

f' = 779 Hz.

But now for finding the wavelength I'm confused.

I know it's λ = v/f'

but is it:

v = 343m/s (the speed of sound in still air)

v= 343+10 m/s (the speed of sound + the speed of the wind)

v= 343+10+20 m/s (the speed of sound + the speed of the wind + the speed of the source)

or something else?

Any help would be much appreciated.

Last edited: