## 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*

*v*

_{p}= v_{t}* (v - v_{0}) / (v)

*where v*

_{t}is the actual frequency and v_{p}is the frequency perceived; v is the velocity of sound in air, and v_{0}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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