Doppler Effect Problem (Very Confusing)

[affank414]

Homework Statement

A pod (group) of dolphins produces sound at the speed of 2050 m/s in cold water at a frequency of 100 kHz. If the pod produces sound at a frequency of 150 kHz, then calculate the velocity of the
pod and the wavelength of the sound.

(Note: The speed of sound in air is 340 m/s).

Relevant Equations

Doppler Effect 2nd case "When observer moving away from source".

P.S. Don't know how to write equation here.

Kindly don't delete the post again, I am a teacher.

The question above mentioned came in Board Exams 2 years ago in our Country. We the group of teachers tried to solve but we found that the problems misses the required information to be solved.

The data still seems to be the case of Doppler's Effect in which source is moving and observer is stationary but still the question lacks the data.

I need review from Senior members on this question. If we're taking this question wrong then let us know the possible logic + solution. Or if the question is contradictory in real then kindly record your comments Thank You.

Kindly don't delete the post again, I am a teacher and discussing this question regarding my students. Or let me know the best place to post this. Thank You.

 

[berkeman]

Welcome to the PF.

It seems like there is enough information there to solve it, as long as you assume that the upshift in frequency is due to the dolphins swimming toward a stationary observer. However, it does seem like that amount of upshift is unreasonable (unless the dolphins are VERY fast). Can you do that calculation for us to show how fast the dolphins would need to be swimming toward a stationary observer to cause the 100kHz --> 150kHz upshift?

 

[BvU]

Don't know how to write equation here.

Look in the guidelines (item 7). Or copy in (e.g. wikipedia) pictures:
$\qquad$

Kindly don't delete the post again, I am a teacher.

The question above mentioned came in Board Exams 2 years ago in our Country.

Do you have a link or an exact rendering of the problem statement ?
$\qquad$

 

[affank414]

Welcome to the PF.

Thank You

It seems there is enough information there to solve it,

How? Do we have any information regarding Observer?

as long as you assume that the upshift in frequency is due to the dolphins swimming toward a stationary observer. However, it does seem like that amount of upshift is unreasonable (unless the dolphins are VERY fast).

Yup still If I solve this question as per "Source of Sound moving towards stationary observer", it comes out to be 683 m/s... Which is unrealistic

Another thing the solution I received from the Board (who had given this question in exam), they applied concept of "Source of Sound Moving Away from Observer"... I mean HOW?

I Asked them that how can we apply this concept but couldn't receive proper reply...

And they had given this question in board exam for class 11 level...

 

[affank414]

Look in the guidelines (item 7). Or copy in (e.g. wikipedia) pictures:
$\qquad$ View attachment 242865

Thanks a lot...

Do you have a link or an exact rendering of the problem statement ?
$\qquad$

Sorry, did you mean exact image from the Question Paper itself?

 

[PeroK]

Yup still If I solve this question as per "Source of Sound moving towards stationary observer", it comes out to be 683 m/s... Which is unrealistic

Another thing the solution I received from the Board (who had given this question in exam), they applied concept of "Source of Sound Moving Away from Observer"... I mean HOW?

For a speed of sound in water of about $2,000 m/s$ the Doppler shift will be small for the sort of speeds that dolphins can swin (maximum $10 m/s$, say). There is no way to get a Doppler shift of 50%.

There's clearly something wrong with the question.

 

[affank414]

For a speed of sound in water of about $2,000 m/s$ the Doppler shift will be small for the sort of speeds that dolphins can swin (maximum $10 m/s$, say). There is no way to get a Doppler shift of 50%.

There's clearly something wrong with the question.

Thanks a lot for your feedback...

For instance if we assume these unrealistic values, does this question still give any information about Observer? Also does it still looks like a problem related to Doppler Effect?

 

[PeroK]

Thanks a lot for your feedback...

For instance if we assume these unrealistic values, does this question still give any information about Observer? Also does it still looks like a problem related to Doppler Effect?

If an observer is not mentioned, then I would assume it is at rest in the water. What else could it be except a Doppler shift? If you changed $150kHz$ to $105kHz$ that might make a difference.

 

[affank414]

If an observer is not mentioned, then I would assume it is at rest in the water. What else could it be except a Doppler shift? If you changed $150kHz$ to $105kHz$ that might make a difference.

And how can we say that source moving towards or away from observer?

 

[PeroK]

And how can we say that source moving towards or away from observer?

That depends how you interpret the question. The question does not make clear what is the natural frequency and the observed frequency. It could be either way round.

 

[affank414]

That depends how you interpret the question. The question does not make clear what is the natural frequency and the observed frequency. It could be either way round.

Thanks a lot for your feedback...

It was given in Grade 11 Physics Exam... If we teachers went into detailed discussion then what about students who were giving exams... They had to suffer a lot in exam...