A factory siren indicating the end of a shift has a frequency of 80 Hz. What frequency is perceived by the occupant of a car traveling towards the factory at 30 m/s? The speed of sound in air is 343 m/s.
A) 83 Hz
B) 85 Hz
C) 87 Hz
D) 89 Hz
E) 91 Hz

Question

Quizplus · Accepted Answer

We can use the formula for Doppler effect: 
f' = f (v+vo)/(v-vs), 
where f is the frequency of the source (80 Hz), v is the speed of sound (343 m/s), vo is the velocity of the observer (0 m/s in this case, since the car is at rest), and vs is the velocity of the source (30 m/s, since the factory is moving towards the car). 
Plugging in the numbers, 
f' = 80 (343+0)/(343-30) = 87 Hz. 
Therefore, the frequency heard by the occupant of the car is 87 Hz.

A Factory Siren Indicating the End of a Shift Has