Physics quiz question: increasing string tension

Physics 205A Quiz 6, fall semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e Multiple-Choice Question 11.5

"Can You Hear Me?: The Longest Tin Can Phone Ever"
Isaac Ravishankara

200 m of mason twine[*] (linear density 9.1×10–4 kg/m) was recently used to make the "longest tin can telephone ever." Increasing the tension in the mason twine would __________ the frequency of waves sent along it.
(A) decrease.
(B) not affect.
(C) increase.
(D) (Not enough information is given.)

[*] "#18 × 425 ft. Orange Polypropylene Twisted Mason Line," homedepot.com/p/Unbranded-18-x-425-ft-Orange-Polypropylene-Twisted-Mason-Line-65375/202957511.

Correct answer (highlight to unhide): (B)

The speed v of transverse waves along the mason twine depends on the tension F and the linear mass density (mass per unit length) (m/L):

v = √(F/(m/L)).

Thus increasing the tension in the twine would increase the wave speed.

Also the wavelength λ is the parameter that depends on the speed v and source frequency f, which can be varied independently of each other:

λ = v/f.

While increasing the tension F of the twine would increase the speed v (which would increase the wavelength λ), this change in the property of the medium would be independent of the frequency, which is a property of the source. Thus increasing the tension in the twine would have no affect on the frequency of the waves sent along it, which depends on the source of the waves, and not the medium.

Sections 70854, 70855, 73320
Exam code: quiz06eAg7
(A) : 10 students
(B) : 35 students
(C) : 19 students
(D) : 0 students

Success level: 55%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75

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