Physics clicker question: sound wave frequency

Physics 5A, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 11.52(c)

Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the end of their learning cycle:

[0.6 participation points.] Consider a cello string, which vibrates at its fundamental frequency of 65.4 Hz. The velocity of sound waves in air is 340 m/s. The frequency of sound waves created by the cello string is:
(A) lower than 65.4 Hz.
(B) equal to 65.4 Hz.
(C) higher than 65.4 Hz.
(D) (I'm lost, and don't know how to answer this.)

Sections 4987, 4988
(A) : 2 students
(B) : 12 students
(C) : 15 students
(D) : 0 students

Correct answer: (B)

While the wavelengths of the string standing wave and sound wave will be different, their frequencies must be the same. It is instructive to think about the fact that the string is vibrating transversely 65.4 times per second, and it is "slapping" the air (and thus generating a sound wave) with this same frequency.