Apparent Frequency (Doppler) Problem

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    Doppler Frequency
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Homework Help Overview

The problem involves the Doppler effect, specifically relating to the change in frequency observed as a driver approaches and then moves away from a parked car's alarm. The original frequency of the alarm is given as 952 Hz, and the speed of sound is noted as 343 m/s. The observed frequency change is 97 Hz.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the frequency changes experienced while approaching and moving away from the source. Questions are raised regarding the interpretation of frequency differences and the velocities involved during these phases of motion.

Discussion Status

There is an ongoing exploration of the relationship between the observed frequency changes and the velocities of the observer. Some participants suggest clarifying the frequencies at different points in the motion, while others express confusion about the calculations and interpretations presented.

Contextual Notes

Participants note potential misunderstandings regarding the direction of frequency changes and the specific velocities during the approach and retreat from the sound source. There is mention of the need to consider the frequency differences at both stages of motion.

wallace13
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The security alarm on a parked car goes off and produces a frequency of 952 Hz. The speed of sound is 343 m/s. As you drive toward this parked car, pass it, and drive away, you observe the frequency to change by 97 Hz. At what speed are you driving?


F=Fo [(Vsound- Vobserver)/(Vsound-Vsource)]



(952-97)=855

855= 952 [(343-V)/(343-0)]

V= 34.9485 m/s
 
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I think you're maybe missing a part of the problem...

On your approach, is the frequency is higher, or lower? What about when you're going away from the source?

So, if you notice a difference of 97 Hz, what is it the difference of - the frequencies heard at which points in your travel? What are your velocities during these times, in relation to the source?
 
The question only asks for one speed, and I added the 97 and subtracted 97 and it was still wrong. And its when you are moving away from the parked car, so wouldn't it be subtracted, anyway? And you're solving for the velocity when you're moving away.

So all in all, I have no idea what you just said
 
Okay, let me try again...

What's the frequency when you're aproaching the siren? What's the frequency when you're going away from it? The difference in frequency should be between these two numbers, I think...

Basically, try for something like [ F(approaching) - F(going away) ] = 97 Hz
 
Okay, since 97 is the difference for approaching AND moving away from the car, you only want half the distance. That should solve you problem :)

(Just divide the answer you got by 2)
 

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