Apparent Frequency (Doppler) Problem

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    Doppler Frequency
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SUMMARY

The discussion centers on solving the Doppler effect problem involving a parked car's alarm emitting a frequency of 952 Hz and a change in observed frequency of 97 Hz as the observer drives toward and away from the source. The formula used is F = Fo [(Vsound - Vobserver) / (Vsound - Vsource)], leading to the calculation of the observer's speed at approximately 34.95 m/s. Participants clarify that the frequency increases when approaching the source and decreases when moving away, emphasizing the need to calculate the frequency at both points to determine the correct speed.

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  • Understanding of the Doppler effect and its implications in sound frequency changes.
  • Familiarity with the formula for calculating frequency shifts due to relative motion.
  • Basic knowledge of sound speed in air, specifically at 343 m/s.
  • Ability to perform algebraic manipulations to solve for unknown variables.
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  • Study the Doppler effect in various contexts, including sound and light.
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  • Explore real-world applications of the Doppler effect in fields such as astronomy and radar technology.
  • Practice solving similar problems involving frequency shifts and relative motion.
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Students in physics, educators teaching wave mechanics, and anyone interested in understanding the principles of sound frequency changes due to 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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