Finding velocity of a vehicle using Doppler Shift for sound

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Homework Help Overview

The problem involves determining the velocity of a vehicle using the Doppler effect for sound, specifically analyzing the change in frequency of engine noise as the vehicle approaches and then passes an observer.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the relationship between the observed frequencies and the speed of the source, noting the presence of two unknowns in the equations. There are questions about the correct setup of the equations and the interpretation of frequency changes as the source approaches and recedes.

Discussion Status

Some participants have offered guidance on how to approach the problem by suggesting the use of a system of equations. Others have pointed out potential errors in the formulation of the equations, particularly regarding the expected behavior of frequency as the source approaches the observer.

Contextual Notes

There is a mention of formatting issues with equations, indicating that clarity in communication may be affecting the discussion. Participants are also navigating the implications of the Doppler effect on frequency changes.

nawg04
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Homework Statement



You are standing on the side of the Trans-Canada Highway as the Physicsmobile approaches you. As it approaches, you hear an engine noise of 1100 Hz. After it passes, you hear an engine noise of 950 Hz. How fast was the Physicsmobile travelling?

Homework Equations



FMHlj.png

f2 = apparent frequency
f1 = actual frequency emitted by source
v = speed of sound in air
vs = speed of source

The Attempt at a Solution



I'm not sure how to re-arrange the formula. If I fill in all variables that I'm given, there are still two left blank (vs and f).
 
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You have two unknown values (vs and f1) and two equations - one before and one after it passes you. You can solve this equation system to get vs.
 
Does this look right?
 
nawg04 said:
Does this look right?

Your first equation is wrong. The source approaches the observer, so the frequency should increase. To increase the frequency, the denominator in the equation should decrease.
 

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