Doppler question - find the observed frequency

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Homework Help Overview

The discussion revolves around a Doppler effect problem where the original poster attempts to find the observed frequency of a sound emitted from a falling object. The context involves concepts of motion, sound propagation, and the application of relevant equations.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the calculation of final velocity and the implications of time taken for both the fall and sound travel. Questions are raised regarding the accuracy of the time measurement and the assumptions made about acceleration and units.

Discussion Status

There is an ongoing exploration of the problem with participants providing insights into potential mistakes and clarifying the relationship between the time of fall and the time for sound travel. Multiple interpretations of the time measurement are being considered, and guidance on constructing equations is being offered.

Contextual Notes

Participants note the importance of units and significant figures, as well as the lack of information regarding the distance of the drop, which complicates the calculation of final velocity.

JoeyBob
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Homework Statement
see attached
Relevant Equations
fobs= ((v-vo)/(v-vs))fs
So first I calculate the final velocity by multiplying the time by the acceleration, 9.8, to get 88.2 m/s.

Now I use the equation. (343/(343-(-88.2))*108.3 = 86.1477.

But the answer should be 88.47. What am I doing wrong here?
 

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9.0s is not the time it takes to fall. It is the time it takes to fall plus the time the sound takes to travel to the surface. So the time it takes to fall is less than 9.0s.

Other mistakes are:
- stating the acceleration is 9.8 without giving the unit;
- giving values for the final frequency and the 'official answer' frequency without giving the unit;
- rounding your answer to an incorrect number of significant figures.
 
Steve4Physics said:
9.0s is not the time it takes to fall. It is the time it takes to fall plus the time the sound takes to travel to the surface. So the time it takes to fall is less than 9.0s.

Other mistakes are:
- stating the acceleration is 9.8 without giving the unit;
- giving values for the final frequency and the 'official answer' frequency without giving the unit;
- rounding your answer to an incorrect number of significant figures.
How would one calculate the final velocity then if distance of the drop isn't given?
 
JoeyBob said:
How would one calculate the final velocity then if distance of the drop isn't given?
If the time it takes to fall is T, then the time for the sound to travel back up is (9.0-T).

You can construct 2 equations:
- one equation for the distance covered by the noisemaker in time T;
- one equation for the distance covered by the sound in time (9.0 - T).
Since these 2 distances are the same you get a quadratic equation for T which you solve..
 

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