Doppler effect and relative motion

[trulyfalse]

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

Homework Statement

A whistle with a frequency of 1200 Hz is traveling south at a velocity of 30.0 m/s. You are traveling 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)

Homework Equations

f=(v/v+v_s)f_s

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?

 

[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, v_o and v_s. You will also need to note that the velocity of the observer v_o in the equation you should find will be negative if he's moving away from the source.