Physics doppler effects

[mikexima]

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EK questions 749

If the source of a sound wave and the observer are stationary, and there is a steady wind blowing from the observer to the source, how will the Doppler effect change the frequency?

A) observed frequency will be greater
B) Observed frequency will be smaller
C) there will be no doppler effect
D) change in frequency can not be predicted

I reasoned with myself that because the wind is blowing in the direction opposite to the source, it will reduce the speed of the sound from source and thus decrease the frequency. but according to EK, the correct answer is C. they say the wind blowing is the same as giving the observer and source velocities of same magnitude but opposite direction to the wind. why?

 

[costales]

The Doppler effect arises from the sound waves bunching up one way or the other due to the motion of the sound source relative to the observer. This question says both observer and source are stationary, so there is no relative motion, hence no Doppler effect.

 

[mikexima]

I still dont understand because the next two questions in EK throws me further off.....

750: stationary source, observer moves towards source, wind blows from observer to source.
answer claims that wind magnifies the doppler effect and increases frequency

751: stationary source, observer moves towards source, however this time wind blows from source to observer.
answer claims that wind minimizes the doppler effect and decreases frequency

how does the effect of wind apply in a manner opposite to its direction?

 

[tn4596]

lets say the wind blow 10 m/s
on ur first question, the wind cause the situation as if BOTH the observer and the source is moving 10 m/s in the same direction (of the wind), there is no change in distance between 'em. thus no doppler effect.
on ur 2nd question, say the speed of observer to source is 5 m/s, the effect of the time would be as if the observer is moving 15 m/s and the source is moving 10 m/s. (same direction) there is a decrease in distance between em over time, therefore the frequency will be increase.
third situation, the observer would be moving 5 m/s in one direction, and the source will be moving 10 m/s to the other direction. The distance between them increase over time, therefore frequency decrease.

 

[costales]

The wind, blowing toward the source, will make the bunched up waves sound even more bunched up, so the observed frequency gets even higher. Now if the wind blows in the opposite direction, the wind effect is lessened and the waves won't sound as bunched up, and the observed frequency will be lower.

 

[mikexima]

i think with the help of your explanations I can understand why now with a more thorough explanation.

lets compare situation two and three and estimate that the wind blows at 10 m/s while the observer walks at 5ms.

if there is no wind in situation two, doppler effect would be normal speed of sound plus speed of walker. so it would be (340+5)/(340-0). now if there is wind blowing against source, it reduces the speed of sound so in essence it would be (330+5)/(330-0).... what happens here is that as the numerator and denominator get smaller, the fraction gets bigger. bigger ratio x original frequeny = higher frequency noticed

the exact opposite applies to situation three..

in situation one, since both the observer and source are stationary, it doesnt matter how fast the sound travels because anything over itself will be 1 and the ratio will remain 1 to 1. (340/340) = 1 (300/300) = 1

example:

10/9=1.11
9/8 = 1.125
11/10=1.1