The Doppler Effect
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AP Physics 2 › The Doppler Effect
A stationary buzzer emits a constant tone. An observer moves directly away from the buzzer at constant speed. Compared to when the observer is at rest, the observed frequency is
Lower because the amplitude decreases as the observer recedes
Lower because the observer meets fewer wavefronts per second
Higher because the observer is moving, increasing wave speed
Unchanged because the buzzer frequency does not change
Explanation
This question tests understanding of the Doppler effect. When an observer moves away from a stationary sound source, the observer encounters wavefronts less frequently because they are moving in the same direction as the propagating waves. This reduced rate of wavefront encounters results in a lower observed frequency compared to when the observer is at rest. The source frequency remains constant, but the observed frequency depends on the relative motion. Choice A incorrectly suggests that observer motion changes wave speed, which remains constant in the medium. Remember: observer motion away from source decreases observed frequency, just as source motion away from observer does.
A speaker on a cart emits a steady $500,\text{Hz}$ tone while moving north toward a stationary microphone. Compared to the emitted frequency, the frequency recorded by the microphone is
Higher because the speed of sound is greater in front of the source
Higher because the microphone receives greater amplitude
Unchanged because the source frequency is fixed at $500,\text{Hz}$
Higher because the source moves toward the microphone
Explanation
This question tests understanding of the Doppler effect. When a sound source moves toward a stationary observer (microphone), the wavefronts are compressed in the direction of motion because each successive wave is emitted from a position closer to the observer. This compression causes the microphone to detect more wavefronts per second than the 500 Hz being emitted, resulting in a higher recorded frequency. The speed of sound remains constant in the medium, making choice C incorrect. Choice A incorrectly assumes that the observed frequency must equal the source frequency, ignoring the effect of relative motion. Remember: source motion toward observer always increases observed frequency, regardless of the specific emitted frequency.
A stationary loudspeaker emits a steady tone in still air. An observer runs directly toward the speaker at constant speed. Compared to standing still, the frequency the observer hears is
Unchanged because only source motion causes Doppler shift
Higher because the observer encounters wavefronts more often
Higher because the speaker emits sound waves faster
Higher because running increases the sound amplitude
Explanation
This question tests understanding of the Doppler effect. When an observer moves toward a stationary sound source, the observer encounters wavefronts more frequently than if standing still because they are actively moving into the oncoming waves. This increased rate of wavefront encounters results in a higher observed frequency. The source continues to emit waves at the same rate, so choice B is incorrect. Choice C represents the misconception that only source motion causes Doppler shift, but observer motion produces the same effect on observed frequency. The fundamental principle: relative motion between source and observer determines frequency shift, regardless of which one is moving.
A stationary ambulance siren emits a steady tone as it drives east at constant speed toward a stationary observer. Compared to when the ambulance is far away and not approaching, which statement best describes the pitch the observer hears while it is approaching? (Assume the speed of sound in air is constant.)
Unchanged because only the source frequency sets the pitch
Higher because the speed of sound increases in front of the ambulance
Higher because wavefronts arrive more frequently at the observer
Higher because the sound amplitude increases as it approaches
Explanation
This question tests understanding of the Doppler effect. When a sound source moves toward a stationary observer, the wavefronts are compressed in the direction of motion, causing them to reach the observer more frequently than when the source is stationary. This increased frequency of wavefront arrival results in a higher observed frequency, which we perceive as a higher pitch. The speed of sound remains constant in the medium regardless of source motion, eliminating choice B. Choice C incorrectly assumes that only the source frequency matters, ignoring the effect of relative motion between source and observer. Remember: when source and observer approach each other, observed frequency increases; when they recede, it decreases.
A boat’s horn emits a steady tone while the boat moves directly toward a stationary dock. Compared to the wavelength in front of the boat when it is at rest, the wavelength in front while moving toward the dock is
Unchanged because wavelength depends only on the horn amplitude
Longer because the speed of sound increases in front of the boat
Unchanged because the horn’s frequency is constant
Shorter because successive wavefronts are emitted from closer positions
Explanation
This question tests understanding of the Doppler effect. When a sound source moves toward an observer, successive wavefronts are emitted from positions that are progressively closer together in the direction of motion, causing the wavelength to be compressed (shortened) in front of the moving source. Since wave speed remains constant in the medium and frequency increases for an approaching source, the wavelength must decrease (λ = v/f). Choice D incorrectly assumes wavelength is unchanged because source frequency is constant, but it's the observed wavelength that changes due to source motion. Remember: wavelength compression occurs in front of a moving source, while wavelength stretching occurs behind it.
A drone emits a constant buzzing tone as it flies directly away from a stationary observer at constant speed. Compared to the emitted frequency, the frequency heard by the observer is
Higher because the drone is moving, increasing the wave speed
Lower because the sound amplitude decreases with distance
Unchanged because the observer is stationary
Lower because the source moves away from the observer
Explanation
This question tests understanding of the Doppler effect. When a sound source moves away from a stationary observer, the wavefronts are stretched out because each successive wave is emitted from a position farther from the observer. This stretching causes the observer to encounter fewer wavefronts per second, resulting in a lower observed frequency compared to the emitted frequency. The observer being stationary doesn't prevent the Doppler effect; what matters is the relative motion between source and observer. Choice B represents the misconception that both source and observer must move for frequency shift to occur. The principle: source motion away from observer always decreases observed frequency.
Two observers stand on a sidewalk as a police car with siren on moves east. Observer 1 is east of the car; Observer 2 is west of the car. At one instant, the car is between them and continues east. Which observer hears the higher pitch at that instant?
Observer 1, because the car is moving toward Observer 1
Both hear the same pitch because amplitude is the same on both sides
Observer 1, because sound travels faster to the east
Observer 2, because the car is moving away from Observer 2
Explanation
This question tests understanding of the Doppler effect. The police car moving east creates different frequency shifts for observers in different positions: Observer 1 (east of the car) experiences the car approaching, while Observer 2 (west of the car) experiences the car receding. When a source approaches an observer, wavefronts are compressed, resulting in higher frequency; when it recedes, wavefronts are stretched, resulting in lower frequency. Therefore, Observer 1 hears a higher pitch than Observer 2. Choice B incorrectly suggests that sound speed varies with direction, which is false in uniform air. The key insight: the Doppler effect depends on whether source and observer are approaching or receding from each other.
A stationary siren emits a steady tone in still air. Two observers run: Observer A runs toward the siren; Observer B runs away from it, each at the same speed. Compared to Observer B, the frequency heard by Observer A is
Lower because Observer A reduces the sound speed relative to them
The same because the siren is stationary
Higher because Observer A encounters more wavefronts per second
Higher because Observer A hears a larger amplitude
Explanation
This question tests understanding of the Doppler effect. Observer A running toward the stationary siren encounters wavefronts more frequently, hearing a higher frequency, while Observer B running away encounters them less frequently, hearing a lower frequency. Since both observers run at the same speed but in opposite directions relative to the source, the frequency shift magnitude is the same but opposite in sign. Therefore, Observer A hears a higher frequency than Observer B. Choice C incorrectly assumes that a stationary source produces no Doppler effect, ignoring that observer motion alone can cause frequency shifts. Remember: the Doppler effect depends on relative motion, not absolute motion of source or observer.
A train horn emits a constant tone. The train moves away from a stationary observer at constant speed on a calm day. Compared to when the train is at rest near the observer, the frequency the observer hears while it is moving away is
Lower because the sound intensity decreases with distance
Lower because the speed of sound decreases behind the train
Lower because successive wavefronts reach the observer less frequently
Unchanged because the observer is not moving
Explanation
This question tests understanding of the Doppler effect. When a sound source moves away from a stationary observer, successive wavefronts are emitted from positions that are progressively farther from the observer, causing the wavefronts to be stretched out. This means the observer encounters fewer wavefronts per unit time, resulting in a lower observed frequency compared to when the source is at rest. The speed of sound in the medium remains constant, so choice B is incorrect. Choice C represents a common misconception that observer motion is required for the Doppler effect, but source motion alone is sufficient to cause frequency shift. The key principle: relative motion away from each other always decreases observed frequency.
A police siren emits a steady tone while the patrol car moves toward a stationary pedestrian at constant speed. Compared to the emitted frequency, the pedestrian hears a frequency that is
lower, because the speed of sound decreases in front of the car
higher, because the wavefronts reach the observer more frequently
unchanged, because only the loudness changes with distance
unchanged, because observer motion is required for a Doppler shift
Explanation
This question tests understanding of the Doppler effect. When a sound source moves toward a stationary observer, the wavefronts are compressed in the direction of motion, causing them to reach the observer more frequently than they were emitted. This increased rate of wavefront arrival results in the observer perceiving a higher frequency than the source actually emits. Choice C incorrectly suggests the speed of sound changes, which reflects the misconception that the Doppler effect alters wave speed rather than just the observed frequency. Remember: when source and observer approach each other, observed frequency increases; when they separate, it decreases.