AP Physics 1 Question 365: Answer and Explanation

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Question: 365

3. You are standing on a railroad track as a train approaches at a constant velocity. Suddenly the engineer sees you, applies the brakes, and sounds the whistle. Which of the following describes the sound of the whistle while the train is slowing down, as you hear it?

  • A. Loudness increasing, pitch increasing
  • B. Loudness increasing, pitch constant
  • C. Loudness decreasing, pitch increasing
  • D. Loudness increasing, pitch decreasing

Correct Answer: D



Intensity is inversely related to the square of the separation between source and detector, so the fact that the train is approaching you means the intensity (loudness) will increase. The pitch, however, is dictated by the Doppler effect. In this case, the detector is motionless, but the source is moving toward the detector. This motion results in a higher pitch at the detector. The brakes of the train (which will, of course, reduce the speed of the train) will continually diminish this effect, so the pitch will decrease.