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.

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.

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.

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.

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.

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.

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.

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.

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.

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.