Setting up AP Physics 1 problems in lab: "Student on a Raft"

On the last day of my AP Summer Institutes I ask participants to spend time just playing in lab.  In particular, I ask everyone to set up something related to a published AP Physics 1 problem.  As a result of this activity - at and beyond my institute - you can see

* Which battery drains first? (Frank Prost, Tom Mellin) (2017)

* Energy vs. time for a cart on rough surface (Nadia Lara, Joey Konieczny) (2015)

* Bumpy Track (Zach Widbin) (2016)

* Waves on a vertical string (Walter Keeley) (2016)

A few weeks ago at TCU, Stephen McAliley wanted to set up the multiple choice problem about a student on a raft in a pool, from the official 2014 practice exam.  Summary: A student starts on the right end of a raft, and walks to the left end.  Which of several pictures correctly represents the location of the student and raft relative to the bottom of the pool?*

*I can't post the problem itself for lawyerly reasons.  But this is easily enough detail to understand the elegance of Stephen's experiment that you'll see below.

I think of this and similar problems as not about relative motion, but about center of mass behavior.  The center of mass obeys Newton's laws.  Since no net force acted on the student-raft system, its center of mass did not change speed from rest.  Then the answer to the original multiple choice question is obvious by inspection.  

This seems like an easy enough problem to set up experimentally - fetch me a raft, a student, a pool, and a video camera.  Not so fast, my friend.  The raft's mass needs to be significant compared to the student's.  And while we have a swimming pool on campus, it's not anywhere close to my classroom.  Peter Bohacek has set it up*: if you have a subscription to Pivot Videos, take a look at "Boy on a surfboard".  

*Of course he has.  My niche is setting up classic physics problems scaled to equipment and space in my high school laboratory.  Peter's superpower is setting up classic physics problems at any scale he wants, with whatever equipment he can get his hands on, with high quality video that includes measuring tools.  Imagine I were writing high quality short stories; then Peter is creating the billion-dollar film, complete with epic action sequences and a score by John Williams.

In the video below, you'll see Stephen's elegant in-class tabletop setup.  The "student" is represented by the PASCO constant speed bulldozer.  The "raft" is a bin, underneath which are two PASCO carts on a track.  Friction between the track and the "raft" is minimal; the weights on top of the bulldozer ensure that the bulldozer doesn't slip on the bin.  To get the student to walk across the raft, Stephen turns on the bulldozer.  Watch:

In the original problem, the raft and student were of equal mass, so that the center of mass was located halfway between the student and the raft's center.  In Stephen's classroom version, the bulldozer and the bin/carts are certainly not of equal mass; and I don't know where the center of mass of the bulldozer is.  

The question I'd ask using this video or live setup, then, is "where's the center of mass of the carts-bin-bulldozer system?"