# Days 46-50: Force Labs & Impulse

Physics: Force Labs

This week was the force of gravity lab and the spring force lab. Students have been struggling to draw meaning from labs, so I spent more time than usual on “translating” the line of best fit into physics and on the conclusions portion of our lab template for the gravity lab, which seemed to pay off. On the spring force lab, I tasked students with doing that more independently and encouraged students to use their force of gravity lab for reference. I noticed a lot of groups using Fg, rather than Fs, and N/kg as the slope unit, rather than N/cm, which tells me students were focusing on what the right answer was in the force of gravity lab, rather than why it was the right answer. This fits with some other observations I’ve made this year and tells me I need to keep thinking about how to get students focusing on their process in labs, rather than what should be the correct result.

The other physics teacher and I decided to use the spring force lab to introduce force sensors since we have some more involved labs coming up where they will need to use force sensors, and I was really pleased with how quickly students got comfortable with those as a tool. I think it was really valuable for students to have their first exposure one where the measurements were relatively straightforward. I tend to fall into using low-tech tools until there is a good reason to use probeware, but the downside is students are then learning to use the probeware in labs with relatively complex scenarios or when they need to use multiple probes to measure different things. I need to remember the value in starting to use probes in relatively simple labs where we don’t strictly need the probeware.

AP Physics 1: Impulse

This week we introduced momentum and impulse. We started with the Modeling Instruction cart explosion lab where students launch spring-loaded carts off each other and graph the ratio of the cart’s velocities vs. the ratio of the cart’s masses and establish the idea of momentum. Next, we did video analysis of two linked air pucks to introduce center of mass (based on an article in The Physics Teacher by Taylor Kaar, Linda Pollack, Michael Lerner, and Robert Engles). After that, we looked at the change in velocity of carts as they crashed into force sensors with hoop springs to introduce impulse. I’ve tried a few versions of that lab and have yet to have students collect satisfying data, so have been thinking about what I want to try instead. In the course of my thinking, I realized I don’t think I’m satisfied with the storyline of my unit. I like the cart explosion lab to introduce momentum and the center of mass piece to think about what it means to treat an object as a particle, but I think those activities lead more naturally into conservation of momentum than impulse. I think next year I want to try starting the momentum transfer model with the same two activities, but then go straight into conservation of momentum. One route to impulse from there could be asking what if we change the system, such as looking at just one cart at a time in the cart explosion lab to motivate new tools in our model.

# Days 46-50: Center of Mass & Vector Addition Diagrams

AP Physics 1: Momentum & Impulse

This week, we worked on developing the concepts of momentum and center of mass. We wrapped up a video analysis exercise (based on an article in The Physics Teacher by Taylor Kaar, Linda Pollack, Michael Lerner, and Robert Engles) where students track a system of two air pucks attached by a rod. Students made a lot of nice connections between the observed motion and Newton’s Laws, which lead to a lot of great discussion about what objects to include in a system to make sense of the motion. I also got to feel very cool demonstrating center of mass in an explosion with a track balanced on some blocks.

Students worked on solving problems with vector addition diagrams for balanced forces. We started with an activity I got from Casey Rutherford where students use pipe cleaners to rearrange the vectors on a free-body diagram before moving into problems on paper. A lot of my students struggle with the idea of rearranging the FBD, so starting by literally rearranging the vectors tends to be a useful stepping stone.

# Day 32: Board Meetings, Gravity, Invented Problems

AP Physics: Board Meetings

Students whiteboarded their results for the collisions lab and the center of mass video analysis from last week. Between the two labs, I think students started to see why it might be useful to switch between systems when thinking about momentum.

Physics: Force of Gravity

Students worked on a lab finding the relationship between mass and the force of gravity. Whiteboards are ready and looking good for a board meeting tomorrow!

Chemistry Essentials: Invented Problems

To review phase changes, I had students write their own problems and exchange whiteboards with another group. There were a few groups that had some good conversation as they were writing their problems and there was some good cross-group conversation when comparing answers after working another group’s problem.

# Day 30: Problems, Annotating Graphs, & Boiling Ice

AP Physics: Problems

Students worked on some conservation of momentum problems. When students asked for help, I could tell pretty quickly who had sketched interaction diagrams. Especially now that I’m embedding center of mass, they have become an incredibly powerful tool. I also demonstrated exploding carts on a balanced track and was pleased at how easily students used the center of mass of the system to explain why it stayed balanced, even when the carts had different masses.

Physics: Annotating Graphs

Students whiteboarded their solutions to yesterday’s problems. I’m pleased to see a lot of students starting to make sense of the physical meaning of the graphs. I was surprised by some students who struggled to differentiate between initial velocity and maximum velocity, but I think they were able to clear up their confusion by the end of the hour.

Chemistry Essentials: Boiling Ice

Students recorded temperatures as ice melted and eventually boiled. A lot of students where quick to say their results didn’t make sense when they saw minimal temperature changes during the phase changes, which was great.

# Day 27: Collisions, Graph Stacks, & Linked Balloons

AP Physics: Collisions

Students started collecting data for the momentum before and after a series of different collisions to discover conservation of momentum. Several groups had a lot of trouble with what we meant by before or after the collision, which showed up as trouble both in filling out the table I gave them and in seeing how to place the carts and photogates appropriately. I wonder if having students draw an SOS diagram for the first collision would have helped with that.

Physics: Graph Stacks

We used the motion encoder to check the graphs students drew for objects on ramps earlier this week. Afterward, students started working on translating between our different representations for accelerated motion. A lot of my conversations with students today have me thinking that many of them are memorizing shapes of graphs without understanding what they represent. I need to keep working on ways to help students attach meaning to the shapes.

Chemistry Essentials: Pressure

I put a large balloon and a small one on opposite ends of a PVC pipe, using alligator clips to close both. Students sketched particle diagrams to predict what should happen when I removed the clips; I wish I’d had them write CERs instead to encourage more interpretation of the particle diagrams. After I showed students both balloons stays the same size, I had them do a second round of particle diagrams to explain why. There was some great conversation about pressure, but I think that phase also would have been better served with a CER.

# Day 26: CoM Board Meeting, Breaking Down Problems, & Gas Laws

AP Physics: Center of Mass Board Meeting

We had a board meeting to discuss the results of the video analysis from the last few days. I asked students to write a CER for whether the forces on each system of pucks were balanced or unbalanced. This lead nicely into some conversation about what exactly we mean by the two -puck system. I ended up wishing students had more experience interpreting position vs. time graphs prior to this discussion; we’ve mostly worked with velocity vs. time graphs so far, and the various uncertainties piled up to make it look like the velocity was changing. The position vs. time graphs were much more convincing, but students weren’t as likely to look at those.

Afterwards, we played with the coupled carts from Kaar, Pollack, Lerner, and Engles’ The Physics Teacher article. The graphs were very satisfying.

Physics: Breaking Down Problems

On the last quiz, a lot of students really struggled to interpret the problems, so we spent some time today on how to break down a physics problem. As we discussed how to approach a problem, we took time for students to apply each step to the problems on the most recent quiz.

Chemistry Essentials: Gas Laws

We used the results of yesterday’s simulation to develop the gas laws. I’ve found a lot of my students really struggle with algebra, so, rather than developing equations, we came up with statements using proportional reasoning which students then applied to some problems. Some of my students who really struggled to manipulate the density equation were very successful with today’s gas laws problems, so I think this approach was a success.

# Day 25: Center of Mass, Board Meeting, & Gas Laws

AP Physics: Center of Mass

Students continued yesterday’s video analysis, based on the article by Taylor Kaar, Linda Pollack, Michael Lerner, and Robert Engles in The Physics Teacher. Today, students analyzed the motion of four hover disks linked into a square from several different perspectives. They were a little bothered that it was tricky to spot the center of the square, but I like that we’ll be able to have a conversation about whether there has to be any mass at the center of mass.

Physics: Board Meeting

Students whiteboarded their results from the video analysis the last few days. Framing this around a CER with a more specific question than usual had the desired effect and I saw students keeping much more complete records than usual. Students are continuing to struggle with recognizing the physical meaning of features on the graph, so I need to keep giving students opportunities to work on that.

Chemistry Essentials: Gas Laws Simulation

Students used the PhET Gas Properties simulation to take quantitative data for the ideal gas laws. I think the class would have benefited from a little more discussion prior to using the simulation to set up what we were measuring and why, rather than just giving them an assignment in Google Classroom that told them what to measure. However, students were very successful in recognizing the quantitative relationships I wanted them to see.

# Day 24: Center of Mass, Ramps, & Gas Laws

AP Physics: Center of Mass

Students started a video analysis activity by Taylor Kaar, Linda Pollack, Michael Lerner, and Robert Engles that recently appeared in The Physics Teacher. I gave students a video of two linked hover disks and had students first track one of the disks, then track the center of mass for the system. In their article, the authors say their students resist tracking the disks, wanting to jump straight to the center of mass. My students, however, were very happy to track the motion of the disks, which made for a really satisfying payoff when they saw how much simpler the motion of the center of mass is.

Physics: Ramps

I’ve found a lot of groups are recording pretty incomplete data during labs. I think since groups don’t make much use of their individual results, some of these students aren’t seeing the value in recording that information. To give them a little more purpose, today we had some discussion to identify changes that could affect the motion of a hover disk on a ramp, then tasked them with collecting data to write a CER to answer how the change affects the motion. This will hit some points I wanted to get anyway, while also giving each group their own task using their data.

Chemistry Essentials: Gas Laws

Students made qualitative observations using sealed syringes in water baths. The ice machine in the school is broken, so the cold water tests didn’t work out very well, but we got some great results with hot water. A few groups had some trouble distinguishing between a change in pressure and a change in volume, so I wish we’d spent some time discussing how we could tell when the pressure in the syringe went up prior to the lab. However, by the end of the hour, groups were able to come up with qualitative descriptions of the ideal gas laws.