Doppler effect and wind

[Halcyon32]

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This question confused the heck out of me. The answer is C for the reason that either the source or observer needs to be moving for there to be a doppler effect but the part that confused me is that they also said that the wind is the same as giving the source and observer a velocity that is equal in magnitude but in the opposite direction to the wind. What does that mean? How can wind blowing in one direction give something a velocity in the opposite direction?

 

[maydaymalone]

My $0.02.

Formula for Doppler effect is f(observer) = f(source)* [(Vsound +/- Vobserver)/(Vsound+/- Vsource)] Now if you have no velocity for observer or source, those will be 0 and cancel out. Then you have Vsound/Vsound which also cancels out and you end up with Fobserver = F source --> therefore no doppler change or doppler "effect" has occured here.

Thats just the way I think about it and any feedback would be cool!

 

[Halcyon32]

My $0.02.

Formula for Doppler effect is f(observer) = f(source)* [(Vsound +/- Vobserver)/(Vsound+/- Vsource)] Now if you have no velocity for observer or source, those will be 0 and cancel out. Then you have Vsound/Vsound which also cancels out and you end up with Fobserver = F source --> therefore no doppler change or doppler "effect" has occured here.

Thats just the way I think about it and any feedback would be cool!

I get why there would be no doppler effect if source and observer are both stationary and your explanation makes sense. But my beef with the question is the part about wind creating a velocity in the opposite direction. Let's pretend that the source is moving; given this case, why would the wind give the source and observer a velocity in the opposite direction?

 

[TUVIX]

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the part that confused me is that they also said that the wind is the same as giving the source and observer a velocity that is equal in magnitude but in the opposite direction to the wind. What does that mean? How can wind blowing in one direction give something a velocity in the opposite direction?

The reason it is the same as if both the source and observer were moving in the opposite direction as the wind is because the velocity of the wind is opposite the direction the sound is traveling. Does that make more sense?

 

[Halcyon32]

The reason it is the same as if both the source and observer were moving in the opposite direction as the wind is because the velocity of the wind is opposite the direction the sound is traveling. Does that make more sense?

Actually, yeah that does make sense. But then wouldn't it be the same to say that the sound is slowing down by a velocity equal to the velocity of the wind, since its blowing in the opposite direction?

 

[TUVIX]

Actually, yeah that does make sense. But then wouldn't it be the same to say that the sound is slowing down by a velocity equal to the velocity of the wind, since its blowing in the opposite direction?

I would have thought that too. But based on what the explanation you have said they gave it's the same as if the source and detector were moving with an equal velocity in the same direction. I guess that makes sense. So image that you have a siren on a train car and a detector on the other end of the train car. The only thing I still don't understand about that analogy is that the source, detector, and medium have the same velocity. Maybe someone else can help us understand that part.

I guess the big question is does the velocity of the medium change the velocity of sound in the medium?

EDIT: thought About it more. If you use the wind as your reference and the wind is the the same speed at the source and the observer then it's like there is no velocity difference between the source and the observer. I think that's the reason.

 

[TUVIX]

Like if you are driving a car and you honk the horn you hear it at a certain frequency but if you then speed the car up by 60 mph you hear the same frequency because your movement relative to the car has not changed. That's the reason.

 

[Halcyon32]

Like if you are driving a car and you honk the horn you hear it at a certain frequency but if you then speed the car up by 60 mph you hear the same frequency because your movement relative to the car has not changed. That's the reason.

This is a really confusing topic for me but I think I get it. I'll have to give it a little more thought but you cleared it up pretty well. Thanks a lot!