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
= 700 * (350 - 35) / (350) = 630 Hz
Want more questions? - Click here to check out our MCAT Question A Day archive
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.
Want more questions? - Click here to check out our MCAT Question A Day archive
