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

AI Thread Summary
The discussion focuses on deriving the Doppler effect formula for a stationary observer and a moving sound source. The key formula presented is v_t = v + v_s, where v_t is the total velocity observed, v is the speed of sound, and v_s is the speed of the sound source. It is noted that when the source moves away, the wavelength increases, resulting in a lower observed frequency. A question arises about why the speed of sound remains constant despite the movement of the source, with clarification that sound speed is determined by the medium, not the source's motion. The analogy of a person running on a moving train is used to illustrate the concept of relative velocity in different contexts.
annamal
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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
 
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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.
 
Hello everyone, Consider the problem in which a car is told to travel at 30 km/h for L kilometers and then at 60 km/h for another L kilometers. Next, you are asked to determine the average speed. My question is: although we know that the average speed in this case is the harmonic mean of the two speeds, is it also possible to state that the average speed over this 2L-kilometer stretch can be obtained as a weighted average of the two speeds? Best regards, DaTario
This has been discussed many times on PF, and will likely come up again, so the video might come handy. Previous threads: https://www.physicsforums.com/threads/is-a-treadmill-incline-just-a-marketing-gimmick.937725/ https://www.physicsforums.com/threads/work-done-running-on-an-inclined-treadmill.927825/ https://www.physicsforums.com/threads/how-do-we-calculate-the-energy-we-used-to-do-something.1052162/
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