Solve Doppler Problems: Train & Person

[cyspope]

Homework Statement

A train approaches an observer at 35.0 m/s and passes a person stading on the side of the track. The whistile is sounding at 2.5 kHz. What frequency will the person hear as the train approaches? When it recedes?

Homework Equations

f' = f $$\frac{v \pm v_o}{v \mp v_s}$$

The Attempt at a Solution

f' = 2500 Hz(?)

I don't know what number I should put into the equation.

 

[Delphi51]

Yes, what a miserable formula that is!
It looks a little better here: http://en.wikipedia.org/wiki/Dopplar_Effect
(scroll down 1 screenfull)
Since the receiver is not moving, the formula is
f = fi*V/(V + Vs)
Here V is the speed of sound in air, and Vs is the speed of the source. The speed of the source toward the receiver increases the frequency, so you must make Vs a negative number (dividing by a smaller number makes the result larger).