I solved questions 1-4. HELP ME PLEASE WITH A QUESTION 5! A physics student sits

Question

I solved questions 1-4. HELP ME PLEASE WITH A QUESTION 5!

A physics student sits by the open window on a train moving at 25 m/sec towards the east. Her boyfriend is standing on the station platform, sadly watching her leave. When the train is 9 meters from the station, it emits a whistle at a frequency of 2000 Hz.

1) What frequency does the boyfriend hear?
f = 1864 Hz

2) What frequency does the physics student hear?
f = 2000 Hz

3) A chemistry student (with nothing better to do than ride around on trains all day) is on a west bound train moving at a velocity of 20 m/sec towards the station. Before the two trains pass each other, what frequency will he hear from the train whistle from the physics student's train?
f = 2283 Hz

4) What frequency will the chemistry student hear after his train passes the physics student's train?
f = 1755.4Hz

5) Now, a wind is blowing to the east at 7.5 m/sec. What frequency would the boyfriend hear in this case?

Explanation / Answer

OK, here Vs is 25 m/s away from the receiver because of the motion of the train relative to the ground and, yes, another 7.5 m/s because of the wind is blowing 7.5 m/s relative to the ground.

SO the speed of the whistle relative to the AIR is 25-7.5=17.5 m/s and is positive.

Now what about the boyfriend? The boyfriend is moving 7.5 m/s away from the train, relative to the air. (He is not actually moving, the air is passing by him at 7.5m/s.

The whole relativity can get confusing, but think of yourself traveling along with the wind.

f = [(343 m/s -7.5m/s) / (343 m/s + 17.5 m/s)] *2000 hz

f = 1861.3037 Hz

The whole idea is of relative velocity, make the aire still by adding the velocity of air to the train and boyfriend

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