EK 1001 Phy 758 Doppler effect

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Mar 2, 2009
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Which of the follwoing will result in the greatest increase in the observed frequency of sound?

A. The source moves towards the observed at 30m/s
B. The observer moves towards the source at 30m/s
C. The source moves away from the observer at 30m/s
D. A and B will result in exactly the same frequency change.


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I think the answer would be D.
Moving towards the source and having the source moving towards the observer will increase frequency. With the doppler effect a higher frequency results in a higher pitch. If you take a look at my poorly drawn picture below :p the person at point A will observe a higher frequency because that observer experiences more sound waves per second then the observer at point B.


fd= frequency of observer
fs= frequency of source
v= velocity of sound
vd= frequency of observer
vs=frequency of source

So having the source move towards the observer equates to more sound waves per second, which means a higher frequency. To get a higher frequency you would subtract in the denominator.

Moving towards the source also results in more sound waves per second. So to get a higher frequency you would add in the numerator.

So plugging in numbers you can see that the ratio would be approximately the same



If you used the exact ratio without rounding, A would be the answer.. but for the sake of the MCAT I highly doubt you would ever need to be that specific. Understanding the concept is key.
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