Final answer:
The pitch change of a vehicle as it moves farther away is due to the Doppler effect, where the sound waves are stretched, leading to a perceived lower frequency by the observer.
Step-by-step explanation:
The reason a vehicle's pitch changes as it moves farther away is due to the Doppler effect. This phenomenon occurs because as the vehicle moves away, the sound waves it emits are stretched out, leading to a lower frequency heard by the observer. When a vehicle approaches an observer, the sound waves are compressed, resulting in a higher frequency. As the vehicle passes by, the observer hears a shift from a higher pitch to a lower pitch due to this compression and subsequent stretching of sound waves.
An example of the Doppler effect in action is when an ambulance with a siren drives past you. At first, as it approaches, the pitch of the siren sounds high. As the ambulance passes and moves away, the pitch drops to a lower frequency. This change in pitch of the siren is directly related to the Doppler effect and not to Newton's First Law, friction with the air, or gravity.
If you are moving away from a stationary source of sound or if the source is moving away from you, you perceive a lower frequency because you encounter fewer cycles of the sound wave each second.