# Day 44: Central Forces & Orbit Wrap-Up

AP Physics: Central Forces

Students whiteboarded their results from Monday’s activity. In the computer lab, there was a lot of debate about what forces were acting on the weight, so I had students include an FBD for the weight on their whiteboards, which lead to some fantastic discussion in my second hour. Students had strong opinions on whether or not there should be a force to balance the tension and were quick to offer an argument one way or another. Even better, a number of students who are usually quiet during whole-class discussions put ideas and arguments out there. Along the way, students connected this back to the bowling ball lab, where one of their tasks was to get the bowling ball to travel in a circle using a rubber mallet. This was the first time I really felt like I got to just sit and listen while my students figured out something important together, and it was fantastic. As the class came to a consensus, one student captured his take with the “mind blown” gesture and sound effect.

Earth Science: Orbit Wrap-Up

Students took a few minutes for students to finish their measurements for the look at the orbit of Halley’s Comet, and we discussed the results. Interestingly, when we looked at the total distance to the focal points, the class averages for Halley’s Comet was better than the class averages for Mars. When I played with the data on my prep, I saw that one group got dramatically different results from the rest of the class. Tomorrow, I’d like to take some time to look at that with my students to get at the idea of outliers and how to handle them in science.

# Day 42: Mass & Comet Orbits

AP Physics: Mass

Students finished the lab practical we started on Thursday where they used a modified Atwood’s machine to find an unknown mass. Today, once they had an answer, we checked their result using a spring scale. One of my goals this year is to work on hearing from every student, so I tried posting three questions about the lab (one for each group member). Then, before groups could check their result, I would roll dice to decide which group member had to answer each question. Since there are three students in each group, I was able to hear from every student. Groups did a really nice job of making sure all three group members could answer all three questions, which is exactly what I hoped would happen.

Earth Science: Comet Orbits

Students finished up yesterday’s lab contrasting Mars’ orbit with Halley’s Comet’s orbit. A lot of groups were surprised when they realized that the patterns from Kepler’s Laws worked equally well for both orbits, especially when they used mass as a proxy to compare the areas of two sections that represent equal times. Next time, I might see if I can get my hands on some card stock or other heavier paper to make it a little more convincing that the small differences are negligible. I also will probably re-work my Kepler’s Law introduction to include Halley’s Comet right off the bat, rather than waiting until we introduce comets.

# Day 41: Central Force & Comet Orbits

AP Physics: Central Force

Students used a Direct Measurement Video of a weight on a rotating table to find a relationship between force and tangential velocity. One of the first questions on my activity was to draw a free-body diagram of the weight and determine whether there is any net force. A lot of groups had some great discussion as they tried to rectify the fact that the table rotates with a pretty constant speed with the fact that their free-body diagrams showed a net force on the weight. There were also some groups that asked questions like how the speed of the banana compared to the speed of the weight, bringing them back to a Direct Measurement Video of a rotating disk we’d used earlier in the year.

Earth Science: Comet Orbits

A major theme this week is comparing different kinds of solar system objects, so I decided to take an idea Michael Lerner gave me last week and have students compare the orbit of Halley’s Comet to Mars. I used an ellipse drawer to give students the orbit of both objects, then had them start by making some observations. Today, most groups only had time for the qualitative questions about the two orbits.

# Day 38: Pushing Blocks & Kepler’s 2nd Law

AP Physics: Pushing Blocks

Students worked on a problem I’ve come to really like where three blocks of different masses are being pushed along by a certain force. Based on some of the struggles a few groups were having, I think it would be worthwhile to pause sometime soon to do a model summary. I loved the moment when students figured out how to use the 3rd Law to think about the normal forces between each pair of blocks. The best thing I overheard, though, was a student who said “You need to convince me you’re right! If you can explain your idea and your evidence, then I’ll believe you.”

Earth Science: Kepler’s 2nd Law

Students plotted the position of Mars along its orbit, then cut out some wedges that represent the same amount of time. We used the mass as a stand-in for area to show that an orbiting object sweeps out the same area in the same amount of time. We got really nice results; the class average for the mass was within 0.01 g for the two wedges. I think students lost track of what they were plotting, however, so I need to think about how I can reinforce what the numbers they are plotting has to do with the actual path of Mars.

# Day 37: 3rd Law & Kepler

AP Physics: 3rd Law

On Friday, students predicted how the force on a pair of carts would compare for various collisions. Today, we got out a pair of carts and force sensors to test out their predictions. I got pretty excited when, after seeing the first couple collisions had the same force, one student took another look at her system schema and free-body diagrams, then pointed out both forces are measuring the same interaction, so the magnitude should be consistent. Afterward, we watched Veritasium’s excellent video on the 3rd Law.

Earth Science: Kepler’s Laws

Students drew ellipses and measured the distance between the foci and points on the ellipse to look for patterns. Students were pretty successful at finding the pattern in the distance to the focal points. I tweaked the lab as written to try to have students discover the equal areas law, but I did not account for the fact that the length of the string loops varies a lot. Tomorrow is slated for more time on this lab, so I think I’m going to try and find some data they can use to plot actual positions of one of the planets tomorrow.