## Wednesday, November 14, 2007

### Momentum lab sequence of happiness

In the canon of introductory physics experiments, the conservation of momentum lab is a classic. High school and college instructors have used a variety of techniques to bring conservation of momentum to students through the years.

I recall using skate-wheel dynamics carts and ticker-tape timers to complete the lab. Dragging paper tails from three-wheel carts had limitations, but it got the job done. By the time this lab was done, students had already learned how to analyze the purple-dotted ticker-tapes.

When I implemented the lab at Rio, I used slick Pasco carts and tracks and photogate timers. Details required attention. Students had to know how to use the photogates, timers, and cart flags to get initial and final speeds of the carts. And the timings had to be done close to the event to keep friction from interfering with the results.

As I recall, my early attempts were to do conservation of momentum as an inquiry activity. I later settled on more of a verification approach. It seemed that no matter what method you used, there were always abundant details of the measurement technology.

That hasn't changed. But now I use motion sensors instead of photogates or ticker-tapes. Nothing needs to be added to the carts. And measurements of speed can be made within a fraction of a second before and after the collision or explosion. Students still need practice and guidance to understand and use the speed measurement technology. But this approach strikes me as the most transparent version I've ever done.

The sequence I've written and implemented also involves the use of a spreadsheet. In one activity, students plug numbers into a spreadsheet that does all the calculations for them. In the the final activity, students must enter their own equations into the cells.

It's a nice sequence on a number of levels. But it does take some time. Six class periods to learn the technology, implement it in inelastic collisions, and implement it in explosions. When lab groups finish one activity, they move on to the next one. Some groups may finish early, but others will need every minute. My early-finishers got to compete for the high score on PhET's Lunar Lander. (One student scored an "out-of-this-world" 150!)

Anyway, here's the sequence.

1. Datastudious. This guides students through the the use of Pasco's DataStudio for the purpose of monitoring motion on the track.

2. Crash and Stick: Inelatic Collisions. How to use the carts to create an inelastic collision. How to use motion sensors and DataStudio to monitor an inelastic collision. And how to enter the data into a spreadsheet that then processes and interprets the results.

I know what you're thinking: students should have to program the spreadsheet themselves--this is simply a low-level, "cookbook" lab. Patience... we'll get there. One step at a time.

3. Such Sweet Sorrow: Explosions. Use motion sensors and DataStudio to monitor explosions. This time, you need two motion sensors. And this time, students must program Excel themselves. (See; I told you we'd get there.)