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The Doppler Effect

I understand it and know what to write in my exam but I want to know why it happens.

Surely, the wavelength should only differ at different speeds. Say a train was moving at a constant speed towards an observer, the wavelength of the train's sound would be the same until the train passed where the wavelength would increase by a lot. Surely, the wavelength would only be constantly changing if the object was accelerating or decelerating but this isn't the case.

Also, shouldn't the momentum of the train cause the waves to be compensated for their unusual distributions? So if a train was moving faster, it would mean the waves emitted forwards would be given more kinetic energy from the momentum, causing the wavelength to not differ at all.

Like I said, I know I shouldn't write any of this in my exam and I should just say that it does it but I want to know why my theories are incorrect.
(edited 6 years ago)
The doppler effect is awesome
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Original post by BillyCockell
The doppler effect is stupid. I understand it and know what to write in my exam but I want to know why it happens.

Surely, the wavelength should only differ at different speeds. Say a train was moving at a constant speed towards an observer, the wavelength of the train's sound would be the same until the train passed where the wavelength would increase by a lot. Surely, the wavelength would only be constantly changing if the object was accelerating or decelerating but this isn't the case.


this would be the case if the sound source on the train was approaching straight towards the listener, passed them at zero distance and receded ... but is not observed in reality because at the instant the sound source attempts to pass through your ear you are also crushed by a train.

since you sensibly stand aside from the train track the relative velocity changes continuously as the train nears and passes you because you are off at an angle to it's path.



Also, shouldn't the momentum of the train cause the waves to be compensated for their unusual distributions? So if a train was moving faster, it would mean the waves emitted forwards would be given more kinetic energy from the momentum, causing the wavelength to not differ at all.

Like I said, I know I shouldn't write any of this in my exam and I should just say that it does it but I want to know why my theories are incorrect.


v=fλ
The speed of sound in air doesn't depend on how fast the source of that sound is moving - you won't hear a supersonic aircraft that's coming towards you
Original post by Joinedup
this would be the case if the sound source on the train was approaching straight towards the listener, passed them at zero distance and receded ... but is not observed in reality because at the instant the sound source attempts to pass through your ear you are also crushed by a train.

since you sensibly stand aside from the train track the relative velocity changes continuously as the train nears and passes you because you are off at an angle to it's path.



v=fλ
The speed of sound in air doesn't depend on how fast the source of that sound is moving - you won't hear a supersonic aircraft that's coming towards you


I get your first response but for your second; does sound have a set, consistent speed in air? This would explain it.
Original post by BillyCockell
I get your first response but for your second; does sound have a set, consistent speed in air? This would explain it.


Fairly consistent, but not a universal constant like the speed of light. It changes depending on air pressure and humidity. In normal circumstances it's about 343ms^-1.
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Kyx
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If you need help, PM me :smile:

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