# Homework Help: Waves and Sound -

1. Dec 9, 2013

### harujina

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

Two train whistles have frequencies of 180 Hz in air (v = 344 m/s). When one train is at rest sounding its whistle, a beat frequency of 2 Hz is heard from the moving train which is sounding its whistle as well. What is the moving train's speed if it is approaching the station?

2. Relevant equations

fbeat = l fo - f1 l

fo = fs (v + vo / v - vs) ; [source is approaching the observer]

fo: observer
fs: source

3. The attempt at a solution

v = 344 m/s
fs = 180 Hz
fo = 182 Hz [fo = fbeat + fs; = 2 Hz + 180 Hz]
vo = 0
vs = ?

fo = fs (v + vo / v - vs) ; [source is approaching the observer]

I'm trying to rearrange this formula to isolate vs but for some reason I can't.
Where do I start... I've tried everything.

And sorry about all the questions, I'm having some significant trouble with this unit in physics...

2. Dec 9, 2013

### MrAnchovy

The first step is to multiply both sides by (v - vs). Can you see why? What is the next step?