Physics quiz question: displacement vector

Physics 205A Quiz 2, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 2.8

[Version 1]

[3.0 points.] An object is initially located at x = –3.40 m, and then moves to a final location at x = +0.55 m. The displacement of this object is:
(A) –3.95 m.
(B) –2.85 m.
(C) +2.85 m.
(D) +3.95 m.

Correct answer: (D)

Displacement is the final position minus the initial position:

delta(x) = x_final - x_initial = (+0.55 m) - (-3.40 m) = +3.95 m.

Response (A) is x_initial - x_final; response (B) is x_initial + x_final; while response (C) is -(x_initial + x_final).

Student responses
Sections 70854, 70855
(A) : 0 students
(B) : 0 students
(C) : 1 student
(D) : 23 students

[Version 2]

[3.0 points.] An object is initially located at x = +0.55 m, and then moves to a final location at x = –3.40 m. The displacement of this object is:
(A) –3.95 m.
(B) –2.85 m.
(C) +2.85 m.
(D) +3.95 m.

Correct answer: (A)

Displacement is the final position minus the initial position:

delta(x) = x_final - x_initial = (-3.40 m) - (+0.55 m) = -3.95 m.

Response (D) is x_initial - x_final; response (B) is x_initial + x_final; while response (C) is -(x_initial + x_final).

Student responses
Sections 70854, 70855
(A) : 13 students
(B) : 3 students
(C) : 0 students
(D) : 7 students

"Difficulty level": 76%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.39