# Homework Help: Doppler effect and relative motion

1. Nov 30, 2011

### trulyfalse

Hello everyone! I need some help with a Doppler effect/relative motion question.

1. The problem statement, all variables and given/known data
A whistle with a frequency of 1200 Hz is travelling south at a velocity of 30.0 m/s. You are travelling north away from the whistle at a speed if 18.0 m/s. If the speed of sound is 340 m/s, what is the apparent frequency of the whistle as heard by you? (Answer: 1044 Hz)

2. Relevant equations
f=(v/v+vs)fs

3. The attempt at a solution
Relative to myself, the train is moving at a speed of 48 m/s south and therefore away from me. Using the aforementioned equation I calculated frequency at 1051 Hz. Is the answer key wrong or am I missing something?

2. Dec 1, 2011

### Stonebridge

The equation you have used is incorrect for the case when both source and observer are moving. Can you check in your notes or book for the correct one? It will have both speeds in it, vo and vs. You will also need to note that the velocity of the observer vo in the equation you should find will be negative if he's moving away from the source.