Doppler equation not making sense to me

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In summary, the final frequency of a sound will increase by a factor of 10 if the source is moving towards the observer and will only increase by a factor of 1.9 if the source is moving towards the observer.
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PsychonautQQ
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Homework Statement


So let's say a source is moving towards an observer at a speed of .9c where c is the speed of sound. the original frequency is 1 Hz.

final frequency = (v(sound) / ((v(sound)-v(source))) * initial frequency
so the final frequency would increase by a factor of 10.

Now let's say that the observer was moving towards the source.

final frequency = (v(sound) + v(observer))/ (v(sound)))intial frequency
where in this case the final freuqnecy would only increase by a factor of 1.9Hz.

Why am I getting different answers? Why does it matter whether the source is moving towards the observer or the observer towards the source?
 
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PsychonautQQ said:
Why does it matter whether the source is moving towards the observer or the observer towards the source?
It's because something quite different is happening depending on whether the source or receiver is moving. From http://physics.bu.edu/~duffy/py105/Doppler.html:Moving receiver:

34a.GIF
Moving source:

34b.GIF
 
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  • #3
PsychonautQQ said:
Why am I getting different answers? Why does it matter whether the source is moving towards the observer or the observer towards the source?
The key is to think about the medium through which the wave is moving. For light, there is no medium, so in that case, you would get the same answer in either case. But sound does have a medium.
 
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Side note: The Doppler effect for light does not exhibit this different behavior depending on whether it's the source or receiver that is moving. You can't even say which one is moving, which one is stationary with light. Moving / stationary with respect to what?

You can say which one is moving, which one is stationary with sound. Sound needs a medium. The speed is measured with respect to the medium, the air in this case. Light doesn't need a medium. There is no luminiferous aether.

Edit: Bruce beat me to it.
 
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hehe, by the skin of my teeth.
 

1. How does the Doppler equation work?

The Doppler equation, also known as the Doppler effect, is a phenomenon that explains the change in frequency of a wave when the source of the wave is in motion relative to the observer. It is commonly observed in sound and light waves, and can be calculated using the equation: f' = f(v +/- vo)/ (v +/- vs), where f' is the observed frequency, f is the original frequency, v is the speed of the wave, vo is the observer's velocity, and vs is the source's velocity.

2. Why does the Doppler equation not make sense to me?

The Doppler equation can be confusing because it involves many variables and calculations. It also requires a basic understanding of wave physics and relative motion. If you are having trouble understanding the equation, it may be helpful to review these concepts or seek assistance from a teacher or tutor.

3. What is the difference between the Doppler equation for sound and light?

The Doppler equation for sound and light are essentially the same, but they use different values for the speed of the wave. The speed of sound is much slower than the speed of light, so the equation will produce different results for each.

4. Can the Doppler equation be applied to all types of waves?

Yes, the Doppler equation can be applied to any type of wave, as long as the wave has a known frequency and the source and observer are in relative motion. It is commonly used in the study of sound and light waves, but can also be applied to other types of waves, such as water waves or radio waves.

5. What are some real-life applications of the Doppler equation?

The Doppler equation has many practical applications in various fields, such as astronomy, meteorology, and medical imaging. It is used to measure the speed and direction of objects in space, to track weather patterns, and to create images of the inside of the human body. It is also used in everyday technologies, such as radar and sonar systems.

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