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Day 64: Collision Video Analysis



AP Physics C: Students used Logger Pro to do video analysis of 2 colliding dynamics carts. Each student in a lab group was responsible for analyzing a different video, then share out results and look for patterns.

The videos are here: Live Photo Physics Colliding Carts. We used videos #31, 32, 34, and 45.

Students are already familiar with collisions from physics last year. So this year, we are focusing on how the center of mass moves and how the carts move relative to the center of mass.


  1. Create a graph showing the position of each cart and the position of the center of mass over time. Find slopes.
  2. Create a second graph showing the position of each cart RELATIVE TO the center of mass over time. Find slopes.
  3. Determine the total momentum of the system before and after the collision.
  4. Determine the total kinetic energy of the system before and after the collision.
  5. Determine the fractional change in internal energy of the system as a result of the collision.


Compare/contrast your results with the others in your group:

  1. Does the velocity of the center of mass remain constant always/sometimes/never?
  2. In the center of mass reference frame, what do you notice about before/after velocities of each cart for elastic and inelastic collisions?
  3. Is momentum conserved always/sometimes/never?
  4. Is kinetic energy conserved always/sometimes/never?

NGSS Science and Engineering Practices:
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking
#7. Engaging in argument from evidence

Day 61: A Last Minute Change of Plans

College-Prep Physics: After Wednesday’s lab to introduce acceleration, I was ready to launch into the unit on constant acceleration. But then I read this modeling listserv email this morning before school:

Teach momentum early.  It allows you to leverage students’ naive conception of “impetus” – the notion that an object carries a force with it as it moves.  In many cases, they have conflated the concepts of force and momentum.

In our progression, we attempt to spiral key concepts in repeatedly.  We begin with constant velocity motion. In addition to the typical tumble buggy, there’s the motion of a hover puck and a glider on an air track to model.  It’s fairly natural to then look for the conditions when we find constant velocity – balanced forces.

In the free particle / balanced force unit, we look at forces as balanced /not balanced, and motion as CV / not CV.  We introduce system schemas, which depict the two-way nature of interactions, and introduce our students to the process of defining a system.  Hover puck and glider come out again as systems for analysis.

Next, we collide gliders on the air track to push the story line forward. We guide their focus to the change in velocity of each glider, and develop the model looking at the pattern of velocity changes observed in different collisions. Following momentum, it’s CA, and then unbalanced forces (CF) to develop N2 and get beyond “CV / not CV”.  Next quarter, we’ll look at forces in collisions, and develop N3 and the impulsive force model.

I like this approach not only because it leverages the student’s naive conceptions, but also because it spirals through core content repeatedly, pulling all of our mechanics work together in the end.

I tried teaching “momentum first” once before, but it was right after constant velocity, not after balanced forces like in the email above. That limited the amount of situations we could analyze, and there was some hand-waving about forces. But right now we are wrapping up balanced forces, so I think moving into momentum now will be more effective than it was previously. So my intended sequence for this year (slightly different than the email above) will now be:

  1. Constant Velocity
  2. Empirical Force Laws and Balanced Forces
  3. Momentum Transfer (Conservation and Impulse)
  4. Constant Acceleration
  5. Unbalanced Forces
  6. Energy Transfer (Conservation and Work)

So after we discussed the results from Wednesday’s speeding up lab, we looked at the forces during collisions. We used Plickers and a modified voting sequence from Preconceptions in Mechanics. Here are my slides:

Then we ended with the colliding carts demo to verify our predictions and models:

NGSS Science and Engineering Practices:
2. Developing and using models
7. Engaging in argument from evidence

Day 60: Another Single-Sentence Lab


AP Physics C: Yesterday I wrote about a single-sentence lab we did in College-Prep Physics. We also did a single-sentence lab in AP yesterday and today:

Determine the rotational inertia of a bowling ball using 2 different and independent methods.

NGSS Science and Engineering Practices:
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking
#7. Engaging in argument from evidence

Day 59: A Single-Sentence Lab

College-Prep Physics: Today I tried an idea from Andrew Morrison (blog, Twitter), which appeared in the November issue of The Physics Teacher: Single Sentence Labs. Andrew writes, “a truly authentic scientific experiment does not come with any instructions.”

So as an introduction to our unit on acceleration, students were given this single-sentence lab: Does a spring rolling down an inclined lab table speed up? Justify your claim with evidence and reasoning.

It was fantastic. Lots of discussion within and between groups about possible experimental designs and analysis.

Some students went the traditional stopwatch and meter stick route:


Others asked for motion detectors:


One group did video analysis:


On Friday, we’ll share our experiments and results.

NOTE: Last year, I did something similar, but used batteries instead of springs. Since the springs are hollow, they have a larger rotational inertia and accelerate more slowly than batteries. It takes about 5 seconds for the springs to travel the length of the 6 foot lab tables at a slight incline (about 3 volumes of Conceptual Physics texts high). I assume PVC pipe cut to pencil length would work well, too.

NGSS Science and Engineering Practices:
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking
#7. Engaging in argument from evidence