Physics clicker question: position vs. time graph fastest speed

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

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 2.14(a) (extended)

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the beginning of their learning cycle, after learning about the "Physics chain of pain" connections between kinematics graphs.

In which time interval(s) does the object have the fastest speed?
(A) t = 0 to t = 2 s.
(B) t = 2 s to t = 3 s.
(C) t = 3 s to t = 4 s.
(D) t = 4 s to t = 5 s.
(E) t = 5 s to t = 6 s.
(F) (More than one of the above choices apply.)
(G) (I'm lost, and don't know how to answer this.)

Sections 70854, 70855
(A) : 0 students
(B) : 0 students
(C) : 7 students
(D) : 21 students
(E) : 17 students
(F) : 1 student
(G) : 0 students

This question was asked again after displaying the tallied results with the lack of consensus, with the following results. No comments were made by the instructor, in order to see if students were going to be able to discuss and determine the correct answer among themselves.

Sections 70854, 70855
(A) : 0 students
(B) : 0 students
(C) : 0 students
(D) : 38 students
(E) : 8 students
(F) : 0 students
(G) : 0 students

Correct answer: (D)

While the time interval from t = 5 s to t = 6 s represents the fastest positive velocity, the t = 4 s to t = 5 s time interval has the steepest slope, corresponding to the fastest velocity magnitude, or speed. Parsing the statement "fastest speed" then, is "steepest |slope|."

Pre- to post- peer-interaction gains:
pre-interaction correct = 46%
post-interaction correct = 83%
Hake, or normalized gain <g> = 68%