So far this year my regular-level physics classes have had three lab assignments. Those are Inelastic Collisions in One Dimension, Elastic Collisions in One Dimension, and Explosions in One Dimension. Each of these involved placing Vernier Motion Detectors at opposite ends of a Vernier Dynamics Track and graphing the motion of a pair of carts as they are put through the various situations described by the lab titles. (To be clear, I have done the same lab just as well with Pasco. I don't advocate Vernier in particular.)

In our labs, students took multiple data runs with varying amounts of mass added to the two carts. The whole goal of this is for students to see that momentum is a useful concept in physics, and that its conservation holds true in many different situations. Hopefully they will then believe it when I tell them that this holds true in any situation where there is no outside force acting on a system.

In each of these labs, the students obviously have to calculate the momentum (mass times velocity) of both carts, both before and after the collision (or explosion, but let's just say "collision" to make it simpler). Students therefore had to have a way to find the velocity of each cart. Earlier this year students learned that the slope of a position vs time graph actually is the velocity of the object being graphed, so that's what I expected they would do. Sure enough, each lab group graphed x vs t for the carts and used the software to determine the linear fit for the time before the collision, and for the time after the collision. They printed out these graphs and stapled them into their notebooks, with the slope clearly labelled.

And then they wrote that the momentum is conserved.

There are no calculations in many of the notebooks. No equations to show whether or not momentum had been conserved in these collisions, and to what degree. There is simply the statement that momentum is conserved. Clearly we've got a disconnect here between my expectations and student performance. It's amazing how exhausting it is to continually stay on top of these things. Students by and large do not want to do this kind of work--it's just a grade. It's very hard to stay excited as an educator when students so clearly see no connection between the work they are asked to do and conclusions they are asked to make, let alone questions on an assessment. To actually connect this with what they think their futures will be?

Today I assigned a step-by-step exercise for the classes in question. For each of the three labs, students were asked to calculate the initial momentum of cart A, and then of cart B, and then the sum of these. And then the final momentum of cart A, and then of cart B, and then the sum of those. And then the percent difference. And then the question: does your data demonstrate that momentum was conserved? It may not have been fun, but it clarified things. Many times students showed me that the data in their notebooks was not sufficient to calculate these values, which means that they could not conclude one way or another about the conservation of momentum. What should we do? Well, frankly, the fact that you realized that you documented the wrong data is more important than the number crunching.

I hope this serves us well in our next lab: Impulse.

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