Monthly Archives: October 2012

Projectile Motion Lab Practicum and Computational Modeling

In my AP Physics B class, I’m reviewing all of the material on the AP exam even though all of the students studied some of this materials last year in either Physics or Honors Physics. When we do have a review unit, I try to keep it engaging for all students by studying the concepts from a different perspective and performing more sophisticated labs.

When reviewing kinematics, I took the opportunity to introduce computational modeling using VPython and the physutils package. I started with John Burk’s Computational Modeling Introduction and extended it with my experiences at Fermilab where computational modeling plays a role in everything from the optics of interferometers to the distribution of dark matter in the galaxy. I then provided students with a working example of a typical projectile motion model and let them explore. I encouraged them to extend the model to have the projectile launched with an initial vertical displacement.

Later that unit, I introduced the lab practicum which was based on a lab shared by my counterpart at our neighboring high school. The goal of the lab was to characterize the projectile launcher such that when the launcher is placed on a lab table, the projectile will hit a constant velocity buggy driving on the floor, away from the launcher, at the specified location. The location would not be specified until the day of the lab practicum. No procedure was specified and students decided what they needed to measure and how they wanted to measure it. I also used this as practice for writing clear and concise lab procedures like those required on the free response section of the AP exam.

All groups figured out that they needed to determine the velocity of the car (which some had done the previous year) and the initial velocity of the projectile. Some groups used a technique very similar to the previous year’s projectile motion lab where a marble is rolled down a ramp and launched horizontally. These groups fired the projectile horizontally from atop the table and measured the horizontal displacement. Groups that calculated the flight time based on the vertical height were more accurate than those that timed the flight with a stopwatch. Another group fired the projectile straight up, measured the maximum height, and calculated the initial velocity. This group was particularly successful. Another group attempted to use a motion sensor to measure the initial velocity of the ball as they fired it straight up. The motion sensor had trouble picking up the projectile and this group’s data was suspect. A couple of other groups fired the projectile at a variety of angles, timed the flight, and measured the horizontal displacement. Some of these groups later realized that they didn’t really need to perform measurements at a variety of angles. After gathering data and calculating the initial velocity of the projectile as a group, I asked the students to practice calculating their launch angle based on a sample target distance. I hadn’t really thought this lab through and didn’t appreciate how challenging it would be to derive an equation for the launch angle as a function of horizontal displacement when the projectile is launched with an initial vertical displacement. It wasn’t until that night that I appreciated the magnitude of this challenge and then realized how this challenge could be used to dramatically improve the value of this lab.

Most students returned the next day a bit frustrated but with an appreciation of how hard it is to derive this equation. One student, who is concurrently taking AP Physics B and AP Physics C, used the function from his AP Physics C text successfully. Another student amazed me by completing pages of trig and algebra to derive the equation. No one tried to use the range equation in the text, which pleased me greatly (the found candy discussion must have made an impact on them). As we discussed how challenging it was to solve this problem, I dramatically lamented, “if only there was another approach that would allow us to solve this complex scenario…” The connection clicked and students realized that they could apply the computational model for projectile motion to this lab. Almost all of the groups chose to use the computational model. One student wrote his own model in Matlab since he was more familiar with that than Python. With assistance, all groups were able to modify the computational model and most were successful in hitting the CV buggy. One group dressed for the occasion:

students ready to launch

Students’ reflections on this lab were very positive. They remarked how they appreciated learning that there are some physics problems that are not easily solved algebraically (they are accustomed to only being given problems that they can solve). They also remarked that, while they didn’t appreciate the value of computational modeling at first, using their computational model in the lab practicum showed them its value. I saw evidence of their appreciation for computational modeling a couple of weeks later when a few of the students tried to model an after-school Physics Club challenge with VPython. For me, I was pleased that an oversight on my part resulted in a much more effective unit than what I had originally planned.

The Game of Life and GridWorld

I love GridWorld, the case study for AP Computer Science. I think it makes the course much more authentic in that students experience developing software in a large (compared to what they are accustomed to) and unfamiliar code base.

One of the first labs that we did was the ActorBox lab published as part of the A+ Computer Science curriculum materials. The students knew almost nothing about GridWorld and nothing about object-oriented programming, but they managed to figure out how to add Actors the the World. I introduced this lab as their first day on the job: “welcome to your new software job, here’s a large code base that you didn’t write and you don’t yet understand, figure out how to …”

Since then we have been learning the basics of Java, and now that we understand conditionals and loops, we can finally write interesting software. For the summative lab, I wanted to revisit GridWorld, emphasize the various phases of software development, and have them write a program that would be memorable. Implementing Conway’s Game of Life in GridWorld seemed to be the perfect fit.

I introduced Conway’s Game of Life last week and presented them with some initial requirements. However, I left a lot of the requirements unspecified. Students enumerated their own requirements on Friday and discussed them with me. I wanted them to realize that they needed to specify such requirements as the grid size, how to represent alive and dead cells, when the program finishes, and the time between generations. Students came up with a variety of ways of specifying these requirements with various levels of user specification. Students then started working on their design (flow charts or pseudocode; we haven’t touched on UML yet). This weekend, the were to enumerate their test cases and they will start implementation this week.

Through all of this, I continuously stressed that their requirements and design should be treated as living artifacts and will evolve as they progress through this lab. (I’m a huge proponent of Agile Software Development Methodologies, but that is a subject for another post.)

Since, at this point in the semester, my students are not that familiar with object oriented design, their solutions will be a functional approach and, most likely, inefficient and implemented with brute force. They will get to apply their newly acquired conditional and loop structures. I also think they will better appreciate the upcoming units related to object-oriented design as a result of this experience. In fact, I’m planning on a future lab to be going back and refactoring their Game of Life program.

Here’s the assignment that I posted:

Game of Life

I think this may be my first computer science, related post… cool.