# Deriving Doppler Effect Frequency w/ Stationary Person & Moving Source

• I
• annamal
In summary: So if the object is moving away from the observer, the wavelength (or distance) of the sound wave will be longer, and the frequency will be lower.
annamal
Can you derive the formula for frequency observed from doppler effect with stationary person and moving sound source away from the person like this:
##v_t = v + v_s## where ##v_t## is the total velocity observed by stationary person from moving sound, v is velocity of sound and ##v_s## is velocity of sound source.
$$v = \lambda f$$
$$f_o = (v + v_s)/\lambda_o$$ where ##f_o## is frequency observed and ##\lambda_o## is wavelength observed

annamal said:
... from doppler effect with stationary person and moving sound source away from the person ...
If moving away, the wavelength will be longer so the frequency will be lower.
Maybe you need to change the sign to (v - vs).

Baluncore said:
If moving away, the wavelength will be longer so the frequency will be lower.
Maybe you need to change the sign to (v - vs).
I guess my question is not very clear. What I am asking is why the velocity of sound is constant regardless of whether the source be moving. Wouldn't the velocity of sound be added to the speed of source? I am likening it to a person running on a moving train. To a stationary viewer the velocity of runner = velocity of runner on train + velocity of moving train.

annamal said:
What I am asking is why the velocity of sound is constant regardless of whether the source be moving. Wouldn't the velocity of sound be added to the speed of source?
Because sound is dependent only on the air. The speed is determined by the medium.
annamal said:
I am likening it to a person running on a moving train. To a stationary viewer the velocity of runner = velocity of runner on train + velocity of moving train.
If you put some air in a box and move the box, the then speed of a sound wave to an external observer IS the speed of the box plus the speed of sound in the box.

vanhees71

## 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 source of the wave.

## 2. How does the Doppler Effect apply to a stationary person and a moving source?

In this scenario, the frequency of the wave will appear to increase as the source moves towards the observer and decrease as the source moves away from the observer. This is known as the "Doppler shift."

## 3. What factors affect the amount of frequency change in the Doppler Effect?

The amount of frequency change depends on the relative velocity between the observer and the source, as well as the speed of the wave. The higher the relative velocity, the greater the frequency change will be.

## 4. Can the Doppler Effect be observed with all types of waves?

Yes, the Doppler Effect can be observed with all types of waves, including sound waves, light waves, and water waves.

## 5. How is the Doppler Effect used in real-world applications?

The Doppler Effect is used in various fields such as astronomy, meteorology, and medical imaging. It is also used in everyday devices such as radar guns and speed cameras to measure the speed of moving objects.

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