Mail Time: How (and especially *when*) I discuss the center of mass in AP Physics 1

I noticed that in the past couple years there've been a lot of center of mass questions on the AP exam. While my best students are capable of figuring these out, I wanted to try driving the point home on an exam with a good short answer justification-style question. I can't seem to make one that I'm satisfied with though. Preferably something that shows the center of mass can not change between two moving objects unless there is net force acting on them. 

I was thinking about an old AP question I saw regarding two balls moving in opposite directions on a moving train. Would it be a good idea to ask students to explain the velocity of the CoM based on the perspective of somebody on and off the train?

Ooh, I know that one!  It's a great question, from the official "practice exam" released in 2014.  

Me, I talk about center of mass very late in the year.  I make sure we're comfortable with the "simple" parts of N2L and momentum conservation first.  Next we do rotation.  I use the term "center of mass" where it's important, but I'm not asking students to truly understand collisions and systems from a center of mass perspective.  

Then, through the Pivot video "marble collides with can and wood block", and a bunch of examples including 2019 P1 #1 (about a velocity-time graph for the center of mass), the students start wrestling with the center of mass concept, and how the CoM obeys Newton's laws.  

The problem you suggest is a great one!  I'd just give it in March or April rather than now.

Definition of acceleration lab, with a spark timer

A correspondent asked me about the "definition of acceleration lab`with dot machine." Is there a specific motion the students are doing? And the "dot machine" is like ticker tape/ strobe diagram? Where does that come in?

This exercise uses the tape timer or spark timer, one that makes 10 dots per second on a long string of paper.  (If you have the 60 dots per second machine, just have students graph every sixth dot!  :-)  )

Here's what I do with the students on lab day:

* Demo the dot machine from the front of the room - 1 minute.

* Divide into groups of 3 or so

* In the back, I have three dot machines [spark timers] set up.  I stand by one of them.  I work with the first group:

   * They adjust the angle of the track between 5 and 30 degrees, and measure that angle.

   * They tape the paper strip to their PASCO cart.  I thread the strip through the machine.

   * I start the machine, they release the cart, they grab their paper strip with dots on.

   * Once they verify that the machine worked, they go away to get position-time data; I start working with the next group.  They re-adjust the track angle, etc.

This takes only two-three minutes per group!  I run the machine because I know what to do, and I get down to business without putzing.  But, if a group doesn't want to wait for me to run the dot machine, they can use one of the other setups.  They see what I'm doing, so they figure things out pretty quickly.  

Once each group has their strip of paper, they make a table of position-time values.  They each get a copy of the response sheet, and they individually make the graph from their table.  The rest of the  response sheet is done as a come-and-show me where I check each part as they complete it.