# Doppler problems.

## 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 $$\frac{v \pm vo}{v \mp vs}$$

## The Attempt at a Solution

f' = 2500 Hz(?????)

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

Delphi51
Homework Helper
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).