The Doppler Effect - AP Physics 2

A siren on an ambulance emits a steady tone while moving toward a stationary pedestrian. Compared to when the ambulance is far away, the pedestrian hears a frequency that is
Answer: higher because wavefronts are compressed as the source approaches This question tests understanding of the Doppler effect. When a sound source moves toward a stationary observer, the wavefronts bunch up in front of the source because each successive wave is emitted from a position closer to the observer. This compression of wavefronts means the observer encounters wave crests more frequently, resulting in a higher observed frequency than the source frequency. Choice C incorrectly assumes that constant source frequency means constant observed frequency, missing that relative motion changes what the observer perceives. The key strategy is to remember that approach increases frequency (compressed waves), while recession decreases frequency (stretched waves).
A train horn sounds continuously as the train moves away from a stationary observer on the platform. Compared to the horn's emitted frequency, the observer hears a frequency that is
Answer: lower, because successive wavefronts arrive less frequently This question tests understanding of the Doppler effect. When a sound source moves away from a stationary observer, the wavefronts are stretched out behind the source, causing successive wavefronts to arrive at the observer less frequently than they were emitted. This decreased rate of wavefront arrival results in the observer perceiving a lower frequency than the horn actually emits. Choice A incorrectly claims the frequency is higher because sound waves speed up, which reflects the misconception that wave speed changes with source motion rather than just the observed frequency. Remember: relative motion changes the rate at which wavefronts reach the observer, not the speed of the waves themselves.
A drone emits a constant tone while flying directly toward a stationary microphone. Compared to the emitted wavelength in still air, the wavelength measured in front of the drone is
Answer: smaller because successive wavefronts are closer together This question tests understanding of the Doppler effect. When a sound source moves toward an observer, the distance between successive wavefronts decreases because each wave is emitted from a position closer to the previous emission point in the direction of motion. This compression results in a smaller wavelength in front of the moving source compared to the wavelength that would exist if the source were stationary. Choice B incorrectly claims wavelength is set only by the source, missing that source motion modifies the spatial distribution of waves. The principle to remember is that wavelength and frequency are inversely related, and source motion affects both.
A car's horn emits a constant tone. The car moves east toward a stationary observer. Compared to an observer west of the car, an observer east of the car hears a frequency that is
Answer: higher because the source moves toward that observer This question tests understanding of the Doppler effect. When a sound source moves toward one observer and away from another, the observers experience opposite frequency shifts due to their different positions relative to the source's motion. The observer east of the car (toward whom the car moves) encounters compressed wavefronts and hears a higher frequency, while the observer west hears stretched wavefronts and a lower frequency. Choice B incorrectly suggests that sound speed varies with direction, when actually the wave speed remains constant in the medium. The principle is that source motion creates asymmetric frequency shifts for observers in different positions.
A stationary lighthouse emits a steady sound signal. A ship moves directly toward the lighthouse at constant speed. Compared to the emitted frequency, the ship detects a frequency that is
Answer: higher because the ship encounters more wave crests per second This question tests understanding of the Doppler effect. When an observer (the ship) moves toward a stationary sound source (lighthouse), the observer encounters wavefronts at a higher rate than if stationary because the ship's motion reduces the time between successive wavefront encounters. This increased encounter rate translates to a higher observed frequency compared to the emitted frequency. Choice C incorrectly claims only source motion matters, missing that the Doppler effect depends on relative motion between source and observer regardless of which moves. The transferable principle is that approach motion always increases observed frequency, whether source or observer moves.
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
Answer: Higher because the source moves toward the microphone 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 buzzer emits a steady tone. An observer runs directly away from the buzzer. Compared to standing still, the observer measures a frequency that is
Answer: lower because the observer encounters wave crests less often 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 increasing the distance between themselves and incoming waves. This decreased rate of wavefront encounters results in a lower observed frequency compared to the source frequency. Choice C incorrectly claims the wavelength in air is unchanged, which is true, but fails to recognize that the observer's motion changes how often they encounter these waves. The strategy to remember is that relative motion affects the rate of wavefront encounters, thus changing observed frequency.
A buzzer on a cart emits a constant tone as the cart moves toward a stationary microphone. Compared to the wavelength in the air behind the cart, the wavelength detected in front is
Answer: shorter, because wavefronts are compressed in front of the source This question tests understanding of the Doppler effect. When a sound source moves toward an observer, the source catches up partially with its own wavefronts, compressing them in the forward direction and reducing the spacing between consecutive wave crests. This compression results in a shorter wavelength in front of the moving source compared to behind it, where wavefronts are stretched apart. Choice B incorrectly claims wavelength depends only on amplitude, which reflects the misconception that wavelength and amplitude are related rather than wavelength and frequency. Remember: source motion physically changes the spacing of wavefronts in the medium, creating different wavelengths ahead and behind.
A stationary speaker emits a constant tone while two observers move: Observer 1 walks toward the speaker, and Observer 2 walks away at the same speed. Which observer hears the higher frequency?
Answer: Observer 1, because moving toward the source increases the observed frequency This question tests understanding of the Doppler effect. When an observer moves toward a stationary source, they encounter wavefronts more frequently, hearing a higher frequency; when moving away, they encounter wavefronts less frequently, hearing a lower frequency. Observer 1, walking toward the speaker, intercepts more wavefronts per second than if stationary, while Observer 2, walking away, intercepts fewer wavefronts per second. Choice C incorrectly claims observer motion doesn't affect frequency, which reflects the misconception that only source motion causes Doppler shifts. Remember: the Doppler effect depends on relative motion—it doesn't matter whether the source, observer, or both are moving.
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
Answer: Lower because successive wavefronts reach the observer less frequently 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.
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?
Answer: Observer 1, because the car is moving toward Observer 1 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 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
Answer: higher, because the wavefronts reach the observer more frequently 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.
A train horn emits a constant tone as the train moves away from a person standing still. Compared to when the train is not moving, the person hears a frequency that is
Answer: lower because wavefronts are stretched as the source recedes This question tests understanding of the Doppler effect. When a sound source moves away from a stationary observer, each successive wavefront is emitted from a position farther from the observer, causing the wavefronts to spread out or stretch. This increased spacing between wave crests means the observer encounters them less frequently, resulting in a lower observed frequency compared to the source frequency. Choice D incorrectly confuses loudness (amplitude) with frequency (pitch), which are independent properties of sound waves. The transferable principle is that relative motion changes frequency, not wave speed or amplitude.
A constant-tone siren is mounted on a car moving toward a listener who is also moving toward the car. Compared to when only the car moves, the listener now hears a frequency that is
Answer: higher, because the relative approach increases wavefront encounter rate This question tests understanding of the Doppler effect. When both source and observer move toward each other, their relative approach speed is the sum of their individual speeds, causing an even greater rate of wavefront encounters than if only one were moving. This increased relative motion results in a higher observed frequency compared to when only the car moves toward a stationary listener. Choice B incorrectly states only source motion matters, which reflects the misconception that observer motion doesn't contribute to the Doppler effect. Remember: the Doppler shift depends on the relative velocity between source and observer—both motions add when they approach each other.
A boat's whistle emits a steady tone while the boat moves north. Observer P is onshore north of the boat; observer Q is onshore south of the boat. Which observer hears the higher pitch?
Answer: Observer P, because the source moves toward P This question tests understanding of the Doppler effect. When a sound source moves north, it approaches observer P (north of the boat) while receding from observer Q (south of the boat), creating opposite frequency shifts for the two observers. Observer P hears compressed wavefronts resulting in higher pitch, while observer Q hears stretched wavefronts resulting in lower pitch. Choice C incorrectly assumes that stationary observers hear the same frequency, ignoring that the source's motion creates different effects based on observer position. The key concept is that source motion toward an observer increases frequency, while motion away decreases it.
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
Answer: Higher because the observer encounters wavefronts more often 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 speaker emits a steady $500\ \text{Hz}$ tone while moving toward a stationary listener at constant speed. Compared to $500\ \text{Hz}$ , the listener hears a frequency that is
Answer: higher, because the speaker's motion decreases the wavelength ahead 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, decreasing the wavelength and increasing the frequency observed. Since frequency and wavelength are inversely related (f = v/λ), the shorter wavelength ahead of the moving speaker results in a higher observed frequency than the 500 Hz being emitted. Choice D incorrectly states the frequency is unchanged because sound speed stays constant, which reflects the misconception that constant wave speed prevents frequency changes. Remember: the Doppler effect changes observed frequency while wave speed in the medium remains constant.
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
Answer: Lower because the source moves away from the observer 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.
A boat's foghorn sounds continuously as the boat moves away from a stationary dock observer. Compared to the emitted sound, the dock observer hears a pitch that is
Answer: lower, because the source is receding from the observer This question tests understanding of the Doppler effect. When a sound source moves away from a stationary observer, the source leaves its wavefronts behind, stretching them out and increasing the spacing between consecutive wave crests. This increased spacing causes the wavefronts to arrive less frequently at the dock, resulting in a lower observed frequency or pitch compared to what the foghorn emits. Choice B incorrectly claims only moving observers experience Doppler shift, which reflects the misconception that observer motion is required rather than just relative motion between source and observer. Remember: any relative motion between source and observer causes a Doppler shift, regardless of which one moves.
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
Answer: Higher because Observer A encounters more wavefronts per second 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.

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