Who's moving towards whom in Doppler effect situations?

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    Doppler Doppler effect
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Discussion Overview

The discussion revolves around the interpretation of the Doppler effect in scenarios involving a skydiver emitting sound while descending at terminal velocity. Participants explore whether the source (the skydiver) is moving towards the observer (the friend on the ground) or vice versa, and how this affects the perceived frequency of the sound. The conversation touches on the implications of the medium (air) for sound waves versus electromagnetic waves.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether the skydiver can be treated as stationary since he is not accelerating, suggesting that the observer is moving towards the source.
  • Others emphasize the importance of considering the speed of sound relative to the medium (air) and how it affects the Doppler effect.
  • There is a discussion about the correct application of the classical and relativistic Doppler shift formulas, with some participants asserting that the formulas may be reversed.
  • One participant points out that sound waves require a medium, while electromagnetic waves do not, which influences the calculations.
  • Some participants express confusion about the roles of the observer and source in the Doppler effect equations, leading to a debate over the correct interpretation of the formulas.
  • A later reply acknowledges a mistake in the application of the formulas, indicating a shift in understanding among participants.

Areas of Agreement / Disagreement

Participants express differing views on the correct interpretation of the Doppler effect in this scenario, particularly regarding the roles of the observer and the source in the formulas. There is no consensus on the correct application of the Doppler shift equations, as some participants believe the formulas are reversed while others maintain their original positions.

Contextual Notes

Limitations include potential misunderstandings of the Doppler effect formulas and the specific conditions under which they apply, as well as the dependence on the medium for sound waves.

azzarooni88
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Hello all, (1st post)

I would like some help with deciding if the source is moving towards the observer or vice versa in Doppler effect situations.

The example that I am confused about is a skydiver holding a constant frequency emitting source during the descent. The skydiver is traveling at terminal velocity. His friend is on the ground and hears a frequency that is higher.
I am wondering why we can't treat the example as the skydiver is stationary since he is not accelerating and that his friend (and the Earth) is moving towards him i.e. Observer is moving towards source?

Source is moving towards observer

Formula: fo = (c + vo)/c) fs

where c=speed of sound

Observer is moving towards source

Formula: fo = (c/c-vs) fs

Cheers
 
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If it's the speed of sound, then you need to consider who is moving relative to the air.
 
PeroK said:
If it's the speed of sound, then you need to consider who is moving relative to the air.
Can you elaborate? My lecturer said it had something to do with the medium (air)
 
azzarooni88 said:
Can you elaborate? My lecturer said it had something to do with the medium (air)

The speed of sound (through air) is independent of the speed of the source.
 
azzarooni88 said:
I am wondering why we can't treat the example as the skydiver is stationary since he is not accelerating and that his friend (and the Earth) is moving towards him i.e. Observer is moving towards source?
The classical Doppler shift formula is given in the rest frame of the medium. The relativistic Doppler shift formula is symmetrical so it doesn’t matter.

http://mathpages.com/rr/s2-04/2-04.htm
 
Thanks. But how can we add the velocity of the observer when its the skydiver that's moving?
 
azzarooni88 said:
The example that I am confused about is a skydiver holding a constant frequency emitting source during the descent.

Is it a source of sound waves or a source of electromagnetic waves? Sound waves require a medium, and so the speed of the medium relative to both the emitter and the receiver has an effect. Electromagnetic waves do not require a medium.
 
Mister T said:
Is it a source of sound waves or a source of electromagnetic waves? Sound waves require a medium, and so the speed of the medium relative to both the emitter and the receiver has an effect. Electromagnetic waves do not require a medium.
Right that makes sense and yes it is a source of sound waves. But when we calculate the velocity of the skydiver, why do we use that value as the velocity of the observer?
 
azzarooni88 said:
Source is moving towards observer

Formula: fo = (c + vo)/c) fs

where c=speed of sound

Observer is moving towards source

Formula: fo = (c/c-vs) fs

Cheers

You have got these the wrong way round, by the way.
 
  • #10
PeroK said:
You have got these the wrong way round, by the way.
I have not actually.
 
  • #11
azzarooni88 said:
I have not actually.

Then I am unable to help you any further.
 
  • #12
PeroK said:
Then I am unable to help you any further.
look up online doppler equation if you don't believe me
 
  • #13
azzarooni88 said:
look up online doppler equation if you don't believe me

You have a formula where the source is moving with a ##v_o## (velocity of observer) in it. And a formula where the observer is moving with a ##v_s## (velocity of source in it). That is the wrong way round!
 
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  • #14
azzarooni88 said:
Source is moving towards observer

Formula: fo = (c + vo)/c) fs
There is a simple sanity check to see whether this is the right formula. Suppose that the source is moving at the speed of sound. The source will exactly keep up with the transmitted signal. All of the peaks and valleys will pile up on top of one another. Wavelength: 0. Frequency: infinite.

The above formula does not have that feature. It merely doubles the received frequency when the velocity of the source is at the speed of the signal.
Observer is moving towards source

Formula: fo = (c/c-vs) fs
This formula (if properly parenthesized) does have the required feature. If you set v=c then the frequency becomes infinite.

This suggests that @PeroK objected correctly that the formulas are reversed.
 
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  • #15
jbriggs444 said:
There is a simple sanity check to see whether this is the right formula. Suppose that the source is moving at the speed of sound. The source will exactly keep up with the transmitted signal. All of the peaks and valleys will pile up on top of one another. Wavelength: 0. Frequency: infinite.

The above formula does not have that feature. It merely doubles the received frequency when the velocity of the source is at the speed of the signal.

This formula (if properly parenthesized) does have the required feature. If you set v=c then the frequency becomes infinite.

This suggests that @PeroK objected correctly that the formulas are reversed.

Okay I agree. My bad. Thank you for fixing it. I've been wrong about a lot recently
 
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