## System of Two Masses – Background

### The Class

Physics 11 is the introductory, calculus-based physics class at Tufts University. This version of the course, with 11 students, is taught by Vesal Dini in a 6-week summer format as part of the Bridge to Engineering Success at Tufts (BEST) program. BEST’s “mission is to retain and graduate underrepresented students, first-generation college students, and students from high schools and regions of the country with low representation within the Tufts School of Engineering.”

The class meets two hours a day each morning. A common sequence of activities in class is a large-group discussion around prelecture or homework questions, followed by small-group work around student-generated questions from the previous night’s prelecture. Whatever the assignment, students are prompted to provide reasoning that underlies their thinking. Discussion on a single problem can last anywhere from 5 to 60 minutes in the large group setting, and even longer when the class is split up into smaller groups.

### This episode

#### Before Class

This is the first day of a unit focused on center of mass. The night before, students watched a prelecture and answered a few checkpoint questions which prompted them both to select an answer and to explain their reasoning. Reviewing their work before class, the instructor, Vesal, noticed a fairly even split in the answers for one question, and so decided to raise that question for discussion in class.

#### In Class

About 40 minutes into the meeting, spent on homework quesetions and another problem, Vesal brought up the “System of Two Masses” question. He highlighted two of the written arguments from the homework: Jared’s (in favor of answer 3, a zero acceleration for the center of mass) and Raul’s (in favor of answer 4, a non-zero acceleration of the center of mass, to the left).

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As Vesal finished reading these two arguments to the class, Korri raised her hand and shared that hearing the arguments had caused her to change her mind (she changed from answer 4 to answer 3). He then re-polled the students and presented the aggregate results of that poll: five for answer 3, five for answer 4, and one undecided.

Vesal then prompted the students to share their reasoning for or against each highlighted argument. About 20 minutes of whole-class discussion ensued, after which Vesal prompted students to “take 30 seconds to think on your own and come up with some reasoning or a question.” The class then, as a whole, discussed the problem for another 20 minutes, which is where we focus attention for this case.