The sound from the whistle of a truck moving at 17.0 m/s toward a cliff reflects from the cliff and is heard by the driver of the truck. When it is parked at the side of the road, the whistle from the
Truck produces sound of frequency 2.267 kHz. As observed by the driver of the truck, what is the
Change in frequency the sound from the whistle due to the reflection? The speed of sound is 343
M/s)
A) 228 Hz
B) 207 Hz
C) 472 Hz
D) 236 Hz
E) 118 Hz

Question

Quizplus · Accepted Answer

The change in frequency observed due to the Doppler effect can be calculated in two steps: first, the frequency increase as the sound moves towards the cliff, and second, the frequency increase as the sound reflects back towards the moving truck. The formula for the observed frequency during the approach is $f' = f \frac{v + v_o}{v - v_s}$, where $f$ is the original frequency, $v$ is the speed of sound, $v_o$ is the observer's speed (0 in the case of the cliff), and $v_s$ is the source's speed. For the reflection, the roles of observer and source are reversed, so the formula becomes $f'' = f' \frac{v + v_s}{v - v_o}$. Substituting the given values and solving gives the change in frequency as 236 Hz.

The Sound from the Whistle of a Truck Moving at 17.0