AP Physics 2 Quiz: The Doppler Effect

What this quiz covers

This quiz focuses on The Doppler Effect, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Physics 2.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Lower because Observer A reduces the sound speed relative to them
2. The same because the siren is stationary
3. Higher because Observer A encounters more wavefronts per second (correct answer)
4. 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.

Question 2

1. lower because the observer encounters wave crests less often (correct answer)
2. the same because the wavelength in the air is unchanged
3. higher because motion away increases the time between crests
4. lower because the sound amplitude decreases when running away

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 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.

Question 3

1. Unchanged because only the source frequency sets the pitch
2. Higher because the sound amplitude increases as it approaches
3. Higher because the speed of sound increases in front of the ambulance
4. Higher because wavefronts arrive more frequently at the observer (correct answer)

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.

Question 4

1. Lower because the speed of sound decreases behind the train
2. Lower because the sound intensity decreases with distance
3. Lower because successive wavefronts reach the observer less frequently (correct answer)
4. 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.

Question 5

1. Unchanged because wavelength depends only on the horn amplitude
2. Longer because the speed of sound increases in front of the boat
3. Unchanged because the horn's frequency is constant
4. Shorter because successive wavefronts are emitted from closer positions (correct answer)

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.

Question 6

1. Observer 1, because the car is moving toward Observer 1 (correct answer)
2. Both hear the same pitch because amplitude is the same on both sides
3. Observer 2, because the car is moving away from Observer 2
4. Observer 1, because sound travels faster to the east

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.

Question 7

1. lower, because the source is now receding from the observer (correct answer)
2. higher, because the speed of sound is greater behind the drone
3. unchanged, because Doppler shift depends only on loudness
4. unchanged, because the drone's distance at that instant is the same

Explanation: This question tests understanding of the Doppler effect. When the drone approaches the observer, wavefronts are compressed, causing a higher observed frequency; when it moves away, wavefronts are stretched, causing a lower observed frequency. At the instant the drone switches from approaching to receding, the observed frequency drops from above the emitted frequency to below it. Choice C incorrectly claims the frequency is unchanged because distance is the same, which reflects the misconception that distance rather than relative motion determines the Doppler shift. Remember: the Doppler effect depends on the component of velocity along the line between source and observer, not the distance between them.

Question 8

1. Unchanged because the buzzer frequency does not change
2. Lower because the amplitude decreases as the observer recedes
3. Higher because the observer is moving, increasing wave speed
4. Lower because the observer meets fewer wavefronts per second (correct answer)

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.

Question 9

1. higher, because the wavefronts reach the observer more frequently (correct answer)
2. lower, because the speed of sound decreases in front of the car
3. unchanged, because only the loudness changes with distance
4. 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.

Question 10

1. higher, because the observer encounters wavefronts more often (correct answer)
2. higher, because the amplitude increases as the cyclist approaches
3. unchanged, because the source is not moving
4. lower, because the sound slows down for a moving observer

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 stationary, because the observer's motion adds to the rate of wavefront encounters. This increased encounter rate causes the cyclist to perceive a higher frequency than the ambulance actually emits. Choice C incorrectly suggests that sound slows down for a moving observer, which reflects the misconception that observer motion changes wave speed rather than just the rate of wavefront encounters. Remember: the Doppler effect occurs whenever there is relative motion between source and observer, regardless of which one moves.

Question 11

1. higher because the observer meets wavefronts more often (correct answer)
2. lower because the observer shortens the wavelength in air
3. the same because the source is stationary
4. higher because the speed of sound increases with observer speed

Explanation: This question tests understanding of the Doppler effect. When an observer moves toward a stationary sound source, the observer intercepts wavefronts more frequently than if standing still because they are actively closing the distance between themselves and incoming waves. This increased rate of wavefront encounters results in a higher observed frequency compared to the source frequency. Choice A incorrectly assumes that only source motion matters, ignoring that the Doppler effect depends on relative motion between source and observer. The key insight is that relative motion toward each other always increases observed frequency, regardless of which object moves.

Question 12

1. Lower because the source moves away from the observer (correct answer)
2. Unchanged because the observer is stationary
3. Higher because the drone is moving, increasing the wave speed
4. Lower because the sound amplitude decreases with distance

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.

Question 13

1. unchanged, because the horn's frequency is constant at the source
2. unchanged, because only moving observers experience Doppler shift
3. higher, because the sound energy decreases with distance
4. lower, because the source is receding from the observer (correct answer)

Explanation: 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.

Question 14

1. the same because frequency depends only on the horn's setting
2. higher because the source moves toward that observer (correct answer)
3. lower because the car's motion reduces the speed of sound ahead
4. higher because the horn is louder in front of the car

Explanation: 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.

Question 15

1. Both the same, because observers are stationary
2. Observer Q, because the speed of sound is higher to the south
3. Observer P, because the source moves toward P (correct answer)
4. Observer Q, because the sound is weaker behind the boat

Explanation: 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.

Question 16

1. the same because the speed of sound is constant in air
2. larger because the source is moving away, spreading wavefronts (correct answer)
3. larger because the sound intensity decreases with distance
4. smaller because the observer is stationary

Explanation: This question tests understanding of the Doppler effect. When a sound source moves away from an observer, successive wavefronts are emitted from positions increasingly distant from each other in the direction opposite to motion, causing the wavelength behind the source to increase. This stretching of wavefronts results in a larger wavelength compared to what would exist if the source were stationary. Choice D incorrectly relates wavelength to sound intensity, when these are independent wave properties - intensity affects amplitude, not wavelength. The key insight is that source motion stretches waves behind and compresses waves ahead.

Question 17

1. smaller because successive wavefronts are closer together (correct answer)
2. smaller because the sound is louder near the microphone
3. greater because the drone pushes the air forward, increasing wave speed
4. the same because wavelength is set only by the source

Explanation: 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.

Question 18

1. Higher because the observer encounters wavefronts more often (correct answer)
2. Unchanged because only source motion causes Doppler shift
3. Higher because running increases the sound amplitude
4. Higher because the speaker emits sound waves faster

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.

Question 19

1. Higher because the source moves toward the microphone (correct answer)
2. Higher because the speed of sound is greater in front of the source
3. Higher because the microphone receives greater amplitude
4. Unchanged because the source frequency is fixed at $500\,\text{Hz}$

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.

Question 20

1. shorter, because wavefronts are compressed in front of the source (correct answer)
2. longer, because the cart increases the speed of sound ahead of it
3. the same, because wavelength depends only on source amplitude
4. the same, because the cart's motion cannot change wavelength

Explanation: 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.