Yesterday, I presented the atomic model of an ideal gas, which students are familiar with from chemistry, from a physics perspective. I didn’t have a paradigm lab to introduce this model. Instead, I shared a GlowScript computational model ported from the VPython hard-sphere model of a gas. We didn’t have time to explore the model today, but will on Monday.
This evening, I read about a much better way to start this unit from Scott Thomas. He had his students develop and explore this computation model over an extended period of times and find the relationship between average kinetic energy and temperature. Next year will be better!
##thermo ##paradigmlab ##computationalmodeling ##setbacks
Today in AP Physics 2, we tied up some loose ends related to fluids before starting thermodynamics. When students performed the lab practicum a couple of days ago, almost all of them placed the cup too far away despite accurate measurements and an accurate computational model. I took this opportunity for us as a class to discuss what sources of error may have been present. When discussing a loss of energy as the stream exits the bottle, I shared a journal article from The Physics Teacher: “Determining the Coefficient of Discharge for a Draining Container.” This article was a great way to finish the fluids unit in that it was accessible to the students and demonstrated the limitations of our fluid dynamics model.
Today, AP Physics 2 students had their first major exam. This is the first time students were directly exposed to the different standards-based grading methodology that I’m trying with AP Physics 2. When the look at the grade book, they will see something like this:
Over time, all of the AP Physics Big Ideas and Enduring Understandings will be represented. To maintain the focus on complex problems that integrate multiple concepts, students taking reassessments will take another entire exam that covers all relevant enduring understandings.
If nothing else, I think reporting learning in this manner will reinforce the big ideas that connect the varied topics that comprise AP Physics 2.
Today, AP Physics 2 students used their computational models for projectile motion of a fluid stream to predict where on the floor to place their cup to catch the water from their 2L bottle with a hole in the side. When I planned this lab practicum over the summer, I was disappointed that we would be developing a computational model for a problem that is fairly easy to solve algebraically. Last year, we used a computational model for a lab practicum where the algebraic solution was beyond most students. I recently remembered advice given to me from a physicist at Fermilab: students must understanding that computational models must be verified against known outcomes before they are used to calculate unknown outcomes. I emphasized this point and encouraged students to check their computational model with an algebraic solution solved by hand. While I mentioned this yesterday, I bring it up again since, today, a student remarked that the computational model was off by 5 cm. He and I were both stumped by this since the projectile motion part of the model is fairly straightforward. Thankfully, another student found the bug. The origin of the model was set to the bottom center of the bottle. The water droplet exiting the hole started a displacement of half the bottle’s width from the origin. The bottle’s width was set to 10 cm. The reported position of the water droplet hitting the floor was based on the origin being located at the center of the bottle and not at the starting position of the water droplet. The student that found the bug quickly fixed it by setting the width of the bottle and enclosed column of water to 0 cm! While totally unplanned, this nicely emphasized the importance of verifying one’s computational model! I have since fixed my model by positioning the initial x position of the water droplet at the origin.
I was so busy today running around watching students demonstrate their prediction and passing out towels, that I forgot to take photos. Thankfully, @anna_kraftson was observing my class and did!
##practicumlab ##fluids ##setbacks ##computationalmodels ##coffeescript ##glowscript
Today, AP Physics 2 students had the first opportunity to modify an existing computational model to prepare for tomorrow’s lab practicum. Tomorrow’s lab practicum is to predict where on the floor to place a cup such that the cup catches the water exiting from a 2L-bottle filled to the specified level and placed the specified distance from the floor. The computation model that I provided implements projectile motion and provides the framework for modeling the bottle and water. Students have to add the physics to calculate the initial velocity of the water. I also emphasized the importance of verifying your model against a known outcome before using it to predict an unknown outcome. Students are using Friday’s quiz as a test case for their model and verifying the prediction of the model against their hand calculations. While this lab practicum doesn’t require the use of a computational model, it is another representation in which the students demonstrate their understanding.
We will see how wet the floor gets tomorrow!
##fluids ##practicumlab ##coffeescript ##glowscript ##chromebooks
In previous years, I’ve used old AP Physics B free-response questions for practice quizzes. With the shift to AP Physics 2, I’m still using these questions, but I’m adding an additional conceptual question to the end that requires students to justify their answer. Today’s practice quiz featured the classic example of a large tank of saltwater with a small drain with a plug and had students solve for the force on the plug, the velocity of the water once the plug breaks free, and the volume flow rate. What I added was:
The saltwater is replaced by an equal weight of freshwater. Indicate whether the speed of the water as it leaves the hole increases, decreases, or remains the same?
Justify your answer.
Few students answered this correctly with the correct justification, and it lead to a great discussion. Some students realized and shared that the velocity of the fluid leaving the drain was only dependent on the height of the fluid in the tank and not the density of the fluid. (They basically derived Torricelli’s theorem.) Few students, however, appreciated that the saltwater was being replaced by an equal weight of freshwater. In order to have an equal weight of freshwater, a greater volume, and therefore greater height, of freshwater must be in the tank. Since the height increases, the velocity of the fluid leaving the drain would also increase.
It’s tough to anticipate the style of the questions on the new AP Physics 2 exam. I hope I’m on the right track.
In AP Physics 2, we whiteboarded the fluid dynamics problems today. This went better in that the questions and discussions were richer than when we whiteboarded the fluid statics problems last week. I expect this was due to two factors: one, students were better prepared; two, the problems were more challenging. In one class, we had a great discussion about why the pressure in an unconstrained stream must be atmospheric pressure. In another, we had a good discussion about how reasoning through a problem conceptually can provide bounds on possible answers that can be used to evaluate the reasonableness of the calculated answer.