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MCAT Question A Day - 3/28/14 - Answer!

A siren sounds with a frequency of 700 Hz. What frequency is heard by a driver in a car moving away from the siren with a uniform velocity of 35 m/s? (The speed of sound in air is about 350 m/s for the ambient temperature.)

A.  720 Hz
B.  630 Hz
C.  540 Hz
D.  790 Hz

The correct answer is (B). The Doppler effect is given by

vp = vt * (v - v0) / (v)

where vt is the actual frequency and vp is the frequency perceived; v is the velocity of sound in air, and v0 is the velocity of the observer.

= 700 * (350 - 35) / (350) = 630 Hz

Choices (A) and (D) can be eliminated immediately because as the source and observer move apart, the frequency drops.

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