After our assessment, we tried some problems and did a little whiteboarding. A few things I learned from my students:
- When students figure out momentum conservation on their own, they keep rediscovering it, and don't use a shortcut. They will calculate impulse on one object. Then they'd use Newton's Third Law to reason that the impulse on the other object is equal and opposite. And then they'd calculate the velocity of the second object. They'll also write momentum conservation in lots of interesting ways, including m₁∆v₁ = -m₂∆v₂.
- On the other hand, they really understand why momentum conservation works. They see that there's no net external force on the system, so they're no impulse on the system. That's why the total momentum doesn't change. I think this is worth it, especially since the biggest drawback is that they solve problems slower, which really isn't a drawback.
- When students get stuck, they seem to stop. When I ask them to draw an SOS diagram (Sketch-Interaction Diagram-Sketch), they start working and then figure out their own way to solve the problem. It seems that they still don't realize how powerful drawing the picture is.