# Who's moving towards whom in Doppler effect situations?

• I
• azzarooni88
In summary, the Doppler effect is a change in frequency of a signal due to the motion of the observer.
azzarooni88
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

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.

PeroK said:
You have got these the wrong way round, by the way.
I have not actually.

azzarooni88 said:
I have not actually.

PeroK said:
look up online doppler equation if you don't believe me

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!

jbriggs444 and berkeman
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.

berkeman and PeroK
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

jbriggs444 and berkeman

## 1. What is the Doppler effect?

The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is commonly observed with sound waves, such as the change in pitch of a siren as an ambulance or police car passes by.

## 2. How does the Doppler effect affect the perception of sound?

The Doppler effect causes a perceived change in the frequency of sound waves, which results in a change in pitch. When the source of the sound is moving towards the observer, the frequency appears higher and the pitch sounds higher. When the source is moving away, the frequency appears lower and the pitch sounds lower.

## 3. What are some real-life examples of the Doppler effect?

Some examples of the Doppler effect in everyday life include the sound of a passing train, the change in pitch of an approaching or receding ambulance siren, and the shift in frequency of a car horn as it drives past.

## 4. How is the Doppler effect used in science and technology?

The Doppler effect is used in various scientific and technological applications, such as weather radars, sonar systems, and medical ultrasound. It is also utilized in astronomy to measure the speed and distance of celestial objects based on the shift in their emitted light.

## 5. How does the direction of movement affect the Doppler effect?

The direction of movement affects the Doppler effect by determining whether the frequency and pitch appear higher (when the source is approaching) or lower (when the source is receding). The magnitude of the change in frequency also depends on the relative velocities of the source and observer.

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