Physics clicker question: Newton's second law

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

Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the beginning of their learning cycle, just after introducing Newton's laws flowcharts.

An object is accelerating at a constant rate in a certain direction. What can be said about the force(s) (if any) acting on it?
(A) The forces (if any) that act on the object add up to zero.
(B) The force(s) that act on the object add up to a (non-zero) net force.
(C) (Nothing can be determined about the forces acting on the object.)
(D) (I'm lost, and don't know how to answer this.)

Sections 70854, 70855
(A) : 15 students
(B) : 23 students
(C) : 0 students
(D) : 1 student

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) : 39 students
(C) : 0 students
(D) : 0 students

Correct answer: (B)

The object is accelerating, which means that Newton's second law applies, meaning that the forces that act on the object add up to a non-zero net force.

Pre- to post- peer-interaction gains:
pre-interaction correct = 59%
post-interaction correct = 100%
Hake, or normalized gain <g> = 100%