PHYSICS · Single correct · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics
Problem
Two cars approach each other on a straight road. Car X travels at 15m/s, and car Y travels at 25m/s. Both drivers sound identical horns.
A passenger in car X hears the horn from car Y.
Which of the following best explains why the passenger hears a higher pitch than the horn’s rest frequency, while the driver of car Y hears no shift in the pitch of their own horn?
(a) Doppler shift depends only on the motion of the source; since car Y (the source) is moving while the driver inside is not, they hear no change, but the passenger in car X does.
(b) The air between the two oncoming cars is compressed, causing sound to travel faster and its frequency to rise for the passenger in car X; inside car Y the speed of sound is unchanged, so no shift is heard.
(c) A Doppler shift occurs when there is relative motion along the line between source and observer; the driver of car Y is at rest relative to their own horn, but the passenger in car X moves toward the source, so they hear a higher frequency.
(d) Because the driver of car Y is moving in the same direction as the sound their horn emits, they chase the wavefronts and hear the original pitch, while the passenger in car X moves against the wavefronts and encounters them more often, producing a higher pitch.