Understanding the Doppler Effect: Relative Velocity Calculation Explained

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Homework Help Overview

The discussion revolves around the Doppler Effect and the calculation of relative velocity between a wave source and an observer. Participants are exploring the implications of relative motion on the perception of wavefronts.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning the formula for relative velocity, specifically whether it should be (v-u) or (v+u) based on the observer's motion. There is an exploration of how the distance between wavefronts changes as the observer moves.

Discussion Status

There is an active exploration of different interpretations regarding relative velocity. Some participants are providing insights into the conditions under which the relative speed is calculated, while others are clarifying the direction of motion and its impact on the perceived speed of waves.

Contextual Notes

Participants are discussing the scenario of an observer moving away from a wave source and the assumptions related to their velocities. There is a mention of specific cases, such as cars moving in the same direction, to illustrate the concept of relative velocity.

kelvin macks
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Homework Statement



can anyone explain why reletive velocity is (v-u), based on my understanding, as the obesrvers moving away, the distance of the f th wavefront and first waavefront increases. so i would say it's v+u . correct me if I'm wrong. thanks.

Homework Equations





The Attempt at a Solution

 

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The distance between wave fronts appears to increase, yes, but that is because the relative speed is also decreasing as the observer accelerates away. If u=v then the wave would never reach you.
 
The wave and the observer are moving in the same direction. Their relative speed is v-u. If they were moving in the opposite direction than the relative speed would be v+u.
 
Last edited:
If two cars travel on the freeway at similar speeds in the same direction the relative velocity between them is small. It's not going to be the sum of their speeds.
 

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