## 20191114

### Physics quiz question: Young's modulus of steel

Physics 205A Quiz 6, fall semester 2019
Cuesta College, San Luis Obispo, CA

A sample of "#4" steel reinforcing bar has a length of 4.9 m and a cross-sectional area of 1.3×10–4 m2. A force of 1.1×105 N is applied to stretch this steel bar by 0.020 m. The Young's modulus of steel is:
(A) 3.5×106 N/m2.
(B) 8.5×108 N/m2.
(C) 4.2×1010 N/m2.
(D) 2.1×1011 N/m2.

[*] webcivil.com/usrcrebar.aspx.

Correct answer (highlight to unhide): (D)

Hooke's law is given by:

(F/A) = Y·(∆L/L),

such that the Young's modulus would be:

(F/A)·(L/∆L) = Y = ((1.1×105 N)/(1.3×10–4 m2))·((4.9 m)/(0.020 m)),

Y = 207,307,692,307.692 N/m2,

or to two significant figures, the Young's modulus for steel is: 2.1×1011 N/m2.

(Response (A) is (F/A)·(∆L/L); response (B) is F/A; response (C) is F/(A·∆L).)

Sections 70854, 70855
Exam code: quiz06co6O
(A) : 5 students
(B) : 2 students
(C) : 2 students
(D) : 43 students

Success level: 83%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50