# EK 1001 Phy 758 Doppler effect

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#### umdnjmed

##### Full Member
10+ Year Member
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.

Thanks

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 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

340/(340-30)=1.097

(340+30)/340=1.097

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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