Physics quiz question: changing wire wave speed

Physics 205A Quiz 6, fall semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 11.6, Multiple-Choice Question 11.5

A wire 10.0 m long is stretched by a tension of 30.0 N, and this length of wire has a mass of 1.90×10^–3 kg.

_____ of the wire would increase the speed of waves along the wire.
(A) Using a shorter length.
(B) Increasing the tension.
(C) Using a longer length.
(D) Decreasing the tension.

Correct answer (highlight to unhide): (B)

Since the wave speed is v = √(F/(m/L)), increasing the tension F will increase wave speed v. Changing the length of wire will not change the linear mass density (m/L), as a shorter or longer length of wire will have a proportionally smaller or larger amount of mass, such that its "thickness" (mass per length) ratio is the same.

Student responses
Sections 70854, 70855
(A) : 5 students
(B) : 33 students
(C) : 1 student
(D) : 1 student

Success level: 82%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.31