What is the relationship between Doppler Effect and frequency?

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Discussion Overview

The discussion revolves around the relationship between the Doppler Effect and frequency, specifically in the context of a train approaching and receding from a stationary observer. Participants explore how to determine the speed of the train based on observed frequency changes and the known speed of sound.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant presents a scenario involving a stationary observer hearing different frequencies (1000 Hz approaching, 800 Hz receding) and seeks to find the speed of the train.
  • The same participant expresses confusion about determining the actual frequency of the train, suggesting that there are two unknowns in the problem.
  • Another participant provides a link to an external resource, which is noted to contain similar information without resolving the original query.
  • A later reply clarifies that there are two equations available for the problem, indicating that one can use the train's speed as a variable in both equations to solve for the unknowns.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to find the actual frequency of the train, and there is ongoing confusion regarding the application of the equations involved in the Doppler Effect.

Contextual Notes

Participants highlight the presence of two unknowns (the speed of the train and the source frequency) and the need for multiple equations to resolve the problem, indicating a potential limitation in the information provided.

studentmom
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I have a question regarding the doppler effect and frequency:

You (a stationary observer) are standing by the railroad tracks and hear a frequency of 1000 Hz as the train approaches... as the train goes away, the frequency changes to 800 Hz. Knowing that the speed of sound is 340 m/s, how fast is the train moving?

Now, I understand that the observed frequency seems different (larger) as the train approaches, and smaller when the train leaves. However, I cannot figure out how to find the actual frequency from the source in order to calculate the speed of the train. It seems to me that I have 2 unknowns. The equation I was trying to use was:
f (observed) = f (source) * (speed of sound/
speed of sound -
speed of train)

Any help would be appreciated!
 
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Thanks, but the information states the same thing that I already knew... the equation is the same, and it offers no explanation of how to figure out the actual frequency of the train's noise...

??
 
studentmom said:
Thanks, but the information states the same thing that I already knew... the equation is the same, and it offers no explanation of how to figure out the actual frequency of the train's noise...
??
You don't have only one equation, you have two equations: in the first one you put vd as the train's speed, in the second you put -vd.
So you have two equations in the two unknowns vd and f (train's frequency).
 

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