AP Physics 1 Question 39: Answer and Explanation
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9. An alarm whose frequency is 400 Hz is dropped out of a third-floor window. The student who drops it measures the frequency with a very sensitive oscilloscope. The measured frequency
- A. appears higher than 400 Hz and the frequency increase as it falls
- B. appears higher than 400 Hz and the frequency decreases as it falls
- C. appears lower than 400 Hz and the frequency decreases as it falls
- D. appears lower than 400 Hz and the frequency increases as it falls
Correct Answer: C
C The velocity of the alarm is away from the detector so the frequency will lower due to the Doppler effect. As the speed increases away from the detector, due to the downward acceleration of gravity, the frequency will get even lower.