Physics
Physics
Problem
A stationary siren produces a sound of frequency 1200 Hz. An observer in a car drives directly away from the siren at 15 m/s. The speed of sound is 340 m/s. Determine the frequency the driver hears.Intuitive explanation
This is the Doppler effect — the shift in frequency when a source and observer move relative to each other. Since the driver is moving away from the stationary siren, the sound waves are stretched out, so fewer wave crests reach the driver per second. The key is using the Doppler formula for a moving observer: the observed frequency depends on the ratio of the observer's effective speed through the medium to the wave speed itself.
Intuitive explanation
This is the Doppler effect — the shift in frequency when a source and observer move relative to each other. Since the driver is moving away from the stationary siren, the sound waves are stretched out, so fewer wave crests reach the driver per second. The key is using the Doppler formula for a moving observer: the observed frequency depends on the ratio of the observer's effective speed through the medium to the wave speed itself.