Doppler effect/ both moving same direction

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Discussion Overview

The discussion revolves around the Doppler effect when both the source and observer are moving in the same direction. Participants explore the implications of different velocities of the source and observer, and how these affect the Doppler equation. The conversation includes theoretical considerations and reference frame shifts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the behavior of the Doppler effect when both the source and observer are moving in the same direction, specifically when their velocities differ.
  • Another participant suggests that if both objects are moving with the same velocity, there would be no Doppler shift as they would be at rest in the center of momentum frame.
  • A participant proposes shifting the reference frame to one where either the source or observer is at rest to simplify the problem to a known situation.
  • It is noted that the movement of air must be considered in the reference frame, which could affect the perceived frequency.
  • There is a discussion about the validity of a specific frequency equation under certain conditions, with a request for clarification on the variables involved.
  • Another participant agrees that the equation can be valid in various situations, provided the definitions of the variables are understood, and reiterates the importance of reference frame shifts.

Areas of Agreement / Disagreement

Participants express differing views on how the Doppler effect applies in this scenario, with no consensus reached on the implications of the velocities or the correctness of the proposed equations.

Contextual Notes

Participants mention the need for clarity on the definitions of variables in the Doppler equation and the influence of the medium's motion, indicating potential limitations in understanding the problem without these details.

QuarkDecay
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What happens if a source and the observer move at the same direction?
Always hear how only one of them doesnot move, or when they move towards each other.

For example. An ambulance moves towards direction A with Va. A biker moves towards direction A as well, with Vb.
What happens to the Doppler equation, if
(a) Va>Vb
(b) Va<Vb
 
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You consider a relative velocity between them. Imagine two objects both moving with the same velocity; you would predict no Doppler shift because they are both at rest in the center of momentum frame, for instance.
 
If you have a good idea of how Doppler Effect works when one is at rest then just shift your reference frame to one in which either of the moving objects is at rest. Then the unknown problem turns into a known situation.
 
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CrazyNinja said:
If you have a good idea of how Doppler Effect works when one is at rest then just shift your reference frame to one in which either of the moving objects is at rest. Then the unknown problem turns into a known situation.

You then have to take into consideration that the air is moving in that frame of reference.
 
pixel said:
You then have to take into consideration that the air is moving in that frame of reference.

Yes true. The velocity of sound with respect to the medium gets adjusted accordingly.
 
So is it wrong if the final frequency becomes f'=[(u+uo)/(u+us)]f, since we consider that the source is moving away from the observer, while the observer moves closer to it (when the velocity of the observer is higher than the source's)
 
QuarkDecay said:
So is it wrong if the final frequency becomes f'=[(u+uo)/(u+us)]f, since we consider that the source is moving away from the observer, while the observer moves closer to it (when the velocity of the observer is higher than the source's)

It would have been better if you had precisely mentioned what "u", "uo" and "us" are in the equation you have written. The equation is valid in all situations provided you know what "u", "uo" and "us" are in the given equation. This also matches with my previous statement about shifting reference frames. In that case the physical values of the aforementioned quantities may change, but the equation still holds.
 

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