Physics quiz question: load-bearing wire

Physics 205A Quiz 6, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 10.4

[3.0 points.] A wire of length 3.00 m with a cross-sectional area of 1.80e-5 m^2 stretches by 0.00250 m when a load of 2.00e+3 N is hung from it. What is the Young's modulus for this wire?
(A) 9.26e+4 N/m^2.
(B) 1.11e+8 N/m^2.
(C) 3.33e+8 N/m^2.
(D) 1.33e+11 N/m^2.

Correct answer: (D)

Hooke's law is (F/A) = Y*(delta(L)/L), so Y = (F*L)/(A*delta(L)). Response (A) is (F*delta(L))/(A*L); response (B) is (F/A), and response (C) is (F*L)/A.

Student responses
Sections 70854, 70855
(A) : 4 students
(B) : 3 students
(C) : 0 students
(D) : 34 students

"Difficulty level": 82%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50