Physics midterm question: fastest speed on vx(t) graph

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 2.11

Shown at right is a v_x(
t) graph of an object traveling along a straight line. The object starts at x = 0 at 
t = 0. When did the object experience its greatest speed?
(A) 0 s < 
t < 2 s.
(B) 2 s < 
t < 4 s.
(C) 4 s < 
t < 6 s.
(D) (More than one of the above choices.)

Correct answer (highlight to unhide): (C)

The object is moving in the +x direction, where it speeds up from 0 m/s to +3 m/s for 0 s < 
t < 2 ; speeds up even further from +3 m/s to +4 m/s for 2 s < 
t < 4 s, and then maintains a velocity of +4 m/s for 4 s < 
t < 6 s. Thus the object has its greatest speed for 4 s < 
t < 6 s. (The slope of this v_x(t) graph is the acceleration a_x of the object, which is +1.5 m/s² from 0 s < 
t < 2 s; a_x = +0.5 m/s² from 2 s < 
t < 4 s; and a_x = 0 m/s² from 4 s < 
t < 6 s.)

Sections 70854, 70855, 73320
Exam code: midterm01p0To
(A) : 15 students
(B) : 3 students
(C) : 53 students
(D) : 1 student

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.28