You are standing on the side of the road and a car with a loud siren goes past you. As it moves away you can expect the sound to become _______ and the wavelength to become_________.

Question

BlackTom AI · Accepted Answer

Consider what happens to sound as the source moves away from you. This involves the Doppler effect and how wavelength relates to pitch.
Option 1: 'higher pitched, longer' This would imply the sound becomes higher in frequency and its waves become more spaced out. In the Doppler scenario of a receding source, frequencies decrease, not increase, so this option is inconsistent with what actually happens.
Option 2: 'lower pitched, shorter' A lower pitch would correspond to a decrease in frequency, but a shorter wavelength would correspond to a higher frequency. These two outcomes conflict with each other, and neither matches the receding-source Doppler pattern.
Option 3: 'lower pitched, longer' This aligns with the Doppler effect for a source moving away: the frequency (pitch) decreases, producing a lower pitch, and the wavelength increases, producing a longer wavelength. The two changes are coherent with the physics of wave propagation when the source recedes.
Option 4: 'the pitch and wavelength should stay the same' In reality, motion of the source relative to the observer alters both frequency and wavelength; nothing about receding motion leaves pitch or wavelength unchanged. This option ignores the Doppler effect.
In summary, the receding car produces a decrease in pitch (lower frequency) and an increase in wavelength (longer waves), which is best captured by the third option.

Physics Honors-Henrikson Unit 9 test part 1

You are standing on the side of the road and a car with a loud siren goes past you. As it moves away you can expect the sound to become _ and the wavelength to become___.

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