AP Physics 1 Question 166: Answer and Explanation

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Question: 166

6. A guitar string creates a sound wave of known frequency. Which of the following describes a correct and practical method of measuring the wavelength of the sound wave with a meterstick?

  • A. Lightly touch the guitar string in the middle such that a single node is created. Measure the length of the string; this is the wavelength.
  • B. Measure the length of the guitar string; this is half the wavelength.
  • C. Adjust the length of a pipe placed near the string so that resonances are heard. Measure the difference between the pipe lengths for consecutive resonances; this is half the wavelength.
  • D. Measure the peak-to-peak distance of the wave as it passes; this is the wavelength.

Correct Answer: C


C-You want the wavelength of the sound wave, not the wave on the guitar. Choices A and B both correctly determine the wavelength of a wave on a guitar string, but these are not the waves you are looking for. Choice D does discuss the sound wave in air, but when is the last time you visually "saw" a sound wave passing you in the air, let alone saw any peaks that you could practically measure with a meterstick? So do the experiment in choice C. Going from one resonance to the next adds half a wavelength to the standing wave in the pipe, regardless of whether the pipe is open or closed.