Wavelength Q: Train Speed 40m/s, Horn 320Hz

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Homework Help Overview

The problem involves a train moving at a constant speed of 40.0 m/s and sounding its horn at a frequency of 320 Hz. Participants are discussing the change in frequency detected by an observer as the train approaches and recedes, as well as the wavelength perceived by the observer during the approach.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants are discussing the application of the Doppler effect to determine frequency changes as the train approaches and recedes. There are attempts to calculate the wavelength using the relationship between velocity, frequency, and wavelength. Some participants question the values used for speed and frequency in their calculations.

Discussion Status

There is an ongoing exploration of the correct application of formulas related to the Doppler effect. Some participants have provided guidance on using the speed of sound instead of the train's speed, while others are clarifying the expected behavior of frequency changes. Multiple interpretations of the problem are being discussed, but no consensus has been reached.

Contextual Notes

Participants are navigating potential confusion regarding the correct values to use in their calculations, including the speed of sound and the frequency of the horn. There are indications of mixed-up answers and a need for clarification on the equations being applied.

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Homework Statement



A train passes a person at a constant speed of 40.0m/s. The train horn is sounded at its characteristic frequency of 320 Hz.

Homework Equations



a) What overall change in frequency is detected by the person as the train moves from approaching to receding?

b) What wavelength is detected by the person as the train approaches?

The Attempt at a Solution



I already solved A. I got a change of -74.2 Hz. 282Hz for approaching and 357.1Hz for receding.

I have absolutely no idea how to do part B. I tried using V = "lambda"f (a.k.a. velocity = wavelength * frequency) but it didn't work.
 
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No, you did the right thing. What didi you use for v? The speed of sound, right? (Not the speed of the train, eh?) What numbers did you use and what answer did you get?
 
Heh, I was using the speed of the train, thanks for that.

however I'm using the speed of sound for V (345 m/s) and for frequency I'm using 282.9, I'm getting the wrong answer.
 
Last edited:
I thought if it's approaching, I use +/- and when receding I use -/+
 
I don't know exactly what equation you are using, but I worked it out and I get different answers than you do for (a).
Think of car horn as you are standing on the side of the street. As it comes towards you, it makes a higher pitch than when it moves away from you. So the frequency must increase as it moves towards you for that to happen. Conversely, for the pitch to be lower as it moves away from you, the frequency must decrease.
 
What equation am I supposed to be using?

I'm using f1 = f((v + v[observer])/(v - v[source of sound])).

I understand the concept of a higher frequency occurring when you are closer to the source, but the formula just isn't working out that way.
 
That's the right equation for when the train is approaching the observer, with v=345m/s and v[observer]=0. You are solving for f1, right?

I don't know, it works for me!
 

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