Day 28: What Happens to Impulse if You Make the System Bigger?

After teaching the impulse-momentum relationship yesterday, I sent them off last night to try a few problems, telling them only that my new type of diagram for showing momentum transfer was an SOS diagram. The two S's stand for sketch, since, in momentum problems, knowing the direction of the momenta is so important. The O is, again, a interaction diagram (or system schema), where any net (non-negligible) external force will cause an impulse. So, instead of writing the impulse-momentum equation as F∆t=m∆v, we wrote it as mvi + Impulse = mvf.

We had a problem in the homework of two colliding railway cars. Without prompting, my students saw two railway cars as one system, and could figure out that the momentum of the whole system would be the sum of the momenta of the constituent parts. After about ten minutes of solidifying their comments, we were off to lab.

The lab I wanted to do wasn't working, because I couldn't get three PASCO probes work on a USB hub. (I'll detail that lab later when I get it to work.) So, instead, I had students set up their own labs. We put motion detectors on each end of the track, and we learned how to track the velocity of each cart on the track. Then, we learned how to have the computer calculate the momentum of each cart and the total momentum of the system. Then I asked them to break the model. Can you find a situation where there was no net external force but the total momentum changed? They couldn't. Here are some pictures I took of their most creative tries of making a cart-cart system that would break the momentum model: