Wave superposition and Doppler effect.

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Homework Help Overview

The problem involves a student running towards a wall while holding a tuning fork, leading to a discussion about the Doppler effect and wave superposition due to reflections from two walls. The student hears a pulsating frequency and seeks to determine the emitted frequency of the tuning fork.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the application of the Doppler effect and wave superposition, with some attempting to relate the beat frequency to the reflections from the walls. Questions arise about how to unify these concepts and apply the Doppler formula correctly.

Discussion Status

Some participants are on the right track with their reasoning, while others are providing hints and guidance about the need to apply the Doppler formula for each reflection. There is an acknowledgment of the complexity introduced by the two walls and the beat frequency concept.

Contextual Notes

Participants are working under the assumption that the speed of sound is 343 m/s and are trying to reconcile their calculations with the expected answer of 137.17 Hz. There is a focus on understanding the relationship between the frequencies involved in the beat phenomenon.

FerN61
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Homework Statement


A student is at some point between two sound reflecting walls. He's holding a tuning fork and runs towards one of the walls at 5m/s. The pulsating frequency he hears is 8Hz. ¿What's the sound frequency emitted by the tuning fork?
Answer: 137.17 Hz.

I know I'm supposed to use doppler effect but I guess I have to consider wave superposition since there are two walls, but I don't know how to unify both.
 
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FerN61 said:
I know I'm supposed to use doppler effect but I guess I have to consider wave superposition since there are two walls, but I don't know how to unify both.
In addition to the Doppler effect, you need to understand beat frequency. (Look that up!)
 
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Beats

Ok so, it is a beat frequency so

f2-f1 = 8

f2= f1+8

using doppler's formula

f1+8=f1((Vsound+5)/Vsound)
Assuming Vsound= 343 m/2

f0=548.8

The answer is supposed to be 137.17... What am I doing wrong?
 
You're on the right track, but there's a bit more to it. The two sounds that are creating the beat frequency are the reflections off the two walls. So you have to find those frequencies. Hint: For each of those reflections you'll need to apply the Doppler formula twice.
 
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