A car horn emits a frequency of 400 Hz. A car traveling at 20.0 m/s sounds the horn as it approaches a stationary pedestrian. What frequency does the pedestrian hear? The speed of sound in air is 343 m/s.

Question

Quizplus · Accepted Answer

When a source of sound is moving towards the stationary observer, the observer hears a higher frequency than the actual frequency emitted by the source. This phenomenon is called the Doppler effect.

The frequency heard by the pedestrian can be calculated using the equation:

f' = f (v±v₀) / (v±vᵢ)

where f is the frequency of the source, v is the speed of sound, v₀ is the speed of the observer (pedestrian), and vᵢ is the speed of the source (car).

In this case, the car is approaching the pedestrian, so we use the plus sign for the velocity terms. We are given that f = 400 Hz, v = 343 m/s, v₀ = 0 m/s (since the pedestrian is stationary), and vᵢ = 20.0 m/s. Plugging in the values, we get:

f' = 400 (343+20) / (343+0)
   = 425 Hz

Therefore, the pedestrian hears a frequency of 425 Hz, which is closest to choice D) 425 Hz.

A Car Horn Emits a Frequency of 400 Hz