9702 · 7.3
Doppler effect for sound waves flashcards
Revision flashcards for Cambridge 9702 Doppler effect for sound waves (syllabus 7.3). Flip, recall, then mark a real past-paper question.
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What is the Doppler effect for sound waves?
The apparent change in a sound wave's frequency due to relative motion between its source and an observer.
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How does frequency change when a sound source approaches an observer?
The observed frequency is higher than the source frequency (a higher pitch).
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How does frequency change when a sound source moves away from an observer?
The observed frequency is lower than the source frequency (a lower pitch).
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State the general formula for the Doppler effect for a moving sound source and stationary observer.
$f_o = \frac{f_s \cdot v}{v \pm v_s}$
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In the Doppler formula, what do $f_s$ and $v$ represent?
$f_s$ is the actual frequency emitted by the source, and $v$ is the speed of sound in the medium.
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Which sign is used for $v_s$ in the Doppler formula when the source moves *towards* the observer?
The minus sign ($-v_s$) in the denominator.
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Which sign is used for $v_s$ in the Doppler formula when the source moves *away* from the observer?
The plus sign ($+v_s$) in the denominator.
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What type of wave is sound, and what is its key oscillation characteristic?
Sound is a longitudinal wave, where particle oscillations are parallel to the direction of energy transfer.
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What are compressions and rarefactions in a sound wave?
Compressions are regions of higher pressure/density; rarefactions are regions of lower pressure/density in the medium.
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Can sound waves travel through a vacuum? Why or why not?
No. Sound waves are mechanical waves that require a material medium (like air or water) to transmit energy through particle vibrations.
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How does the wavelength of a sound wave change for an observer when the source is approaching them?
The observed wavelength decreases. The wavefronts are compressed, leading to a shorter distance between them.
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In the Doppler formula $f_o = \frac{f_s \cdot v}{v \pm v_s}$, what is the key assumption about the observer?
The observer is assumed to be stationary relative to the medium (e.g., the air).
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A source moves away from you. Will the observed frequency be higher or lower than the source frequency, and which sign is used for $v_s$?
The observed frequency will be lower. The plus sign ($+v_s$) is used in the denominator: $f_o = \frac{f_s \cdot v}{v + v_s}$.
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Give one real-world application of the Doppler effect using sound waves.
Doppler ultrasound uses the frequency shift of sound waves reflected from moving blood cells to measure blood flow and velocity in arteries and veins.