Preparing for the LAB FRQ for AP® Physics I
The third Free Response Question on the AP Physics I exam is on “Experimental Design and Analysis”, which I will simply refer to as the LAB FRQ. This is my first year teaching AP Physics (so please take everything with a grain of salt!) but in reading the CED, doing practice FRQs with my students, and looking at patterns in the newest FRQs and in ones marked “High Alignment” in AP Classroom, I see some patterns that are shaping how I approach labs in my class. (Obligatory groan for teaching to the test, however, I think there are some really good skills being embedded in the way these questions are asked that will benefit students in future lab classes or research). This blog post is partly to help me focus and clarify my thinking on this as I make plans for the second half of the course, partly to share my thoughts and approaches, and partly to hear from other experienced AP teachers on how they approach labs and FRQs.
Unpacking this FRQ: What the CED says and patterns in released questions
The LAB FRQ is explained on page 181 of the CED. It is worth 10 points and is expected to take up to 30 minutes. It targets science practice 3.A (write a procedure), requiring students to write a reasonable procedure to test some given scenario using good methods; then they will analyze a similar scenario via graphing (practice 1.B), typically with linearizing, to calculate an unknown (practice 2.B) and/or predict a new value (practice 2.D).
The questions marked “High Alignment” in AP Classroom have a very consistent format (which matches the examples in the CED). They go like this…
Here’s an experimental scenario, usually with a picture.
Describe an experimental procedure to determine X [where X cannot be directly measured easily in this described scenario]. include steps to reduce experimental uncertainty.”
Describe how the data from [the previous part] could be graphed to determine a value for X.
Here’s a twist on that first scenario and some data that was collected based on it.
Indicate which data needs to go on each axis to get a linear graph. (Sometimes a relevant equation is given, sometimes students need to pick out the relevant equation from the equation sheet).
[sometimes implied, sometimes explicit] Add the appropriate data to the data table based on what you said you need to graph.
Plot the data you said you needed with good scales and labels.
Draw a line of best fit.
Calculate an experimental value for X using the line of best fit (typically from the slope).
Low alignment questions also involve deriving an expression, drawing free body diagrams, explicitly listing out what data is to be collected and how to collect it (which is implied in the “Describe a procedure” part, or drawing an experimental setup (which is encouraged but optional in some of the high alignment questions).
The big idea of this question seems to be “take a physical situation and get a numerical value for an unknown value using a linearized graph”.
(Tangent–I got a whole dang physics degree, did a Master’s in physics education, and somehow did not come across this idea of “linearizing” until I went to the AP Summer Institute. Sure, I did tons of data analysis but like, in the real world, with computers. I never had to plot data and calculate a slope by hand in my actual physics degree or research. Like, I get why it could be a useful practice for our kids, in that it helps them connect equations, data, and a physical situation, but will they ever use it again?? Doubtful. End rant.)
So How Should They Answer It? What I see in scoring guides
These are the things I see in all the scoring guides that students need to do for full credit:
For the initial scenario…
List all the relevant measurements and how they’re measured in the procedure.
Have a clear manipulated and responding variable.
Have a procedure that could reasonably be done in a high school lab and actually produce the data they’re looking for.
Say something about uncertainty (“multiple trials”, hint hint.)
Describe a graph that puts one variable on each axis.
Identify the relationship (equation from the equation sheet) that will connect the two variables such that when it's graphed, its slope will give you X.
So for instance, if you were determining g using distance and time, put time squared on x, distance on y, the slope is 1/2g. Or if you were determining an unknown mass, you could put acceleration on x, force on y, and the slope is mass.
For the second scenario with data…
Identify which equation connects the data to the unknowns.
Use that equation to figure out what needs to go on each axis and state the variables WITH UNITS.
Calculate any values needed (for instance, you might need to square all the time values if you’re finding g from distance and time)
Label the graph axes WITH UNITS.
Choose a scale that uses AT LEAST HALF of the available gridlines (if you used less than half, you could have just doubled the spacing). The spacing must be uniform.
Plot the data correctly.
Draw a line of best fit. (Caution: if you have not linearized and it’s not a linear graph, you shouldn’t force a straight line! If it is linear, the line needs to be straight, so students should bring and use a ruler.)
Find the slope of the line, showing the calculation. Tip: don’t use data points as your two points for slope, pick 2 spots where the line neatly crosses gridlines.
Use the value of the slope and the known equation to calculate a value for the unknown and show the calculation. There is some wiggle room in the accepted values, but if your line of best fit sucks, this number won’t come out right.
Connecting it to Labs: Consistency and clarity
Now obviously we can have kids practice the available lab FRQs, but there aren’t many. The better option is to have them use these same skills (writing procedures, graphing and linearizing data, determining an unknown from a graph) in our actual labs. The more opportunities they have to do this in the lab, the more prepared they’ll be for what the exam may throw at them.
In my standard physics classes, I use a standardized lab graphic organizer to scaffold and structure their lab skills, and diving into this FRQ has made me want to create something similar for my AP kids. I would be able to start the year with the exact same skills we need all year (procedure writing, linearization, etc.) but with lots of modeling and guidance, then pull away the supports as we progress and their skills build. Like I talked about in my post about my standard lab organizer, when I can give my kids consistency and predictability in how I want them to record their labs and how I will grade them, they can dedicate more of the mental energy to actually engaging with the science. It also has the benefit of allowing for data collection on specific skills to target future instruction. In the next couple of weeks, I plan on creating a lab graphic organizer for AP with an accompanying rubric. I’ll test it out on my rotation labs in units 5 & 6 and let you know how it goes!!
If I model the organizer off of the structure of the LAB FRQ, I can make it very clear for students how the skills they’re displaying in the lab relate to how they’ll be assessed on the exam. An excellent piece of advice I got at my APSI was to accompany labs with FRQs that are based on a similar scenario. For instance, if you do a lab about determining g, follow it up with one of the released FRQs that determine g to future underscore the connections.
Practicing with Experimental Scenarios: A drill I’m testing out
Because we only have so much time to do labs, I came up with a practice to do every few blocks at the start of class. I let the kids pick a random number, and we analyze a scenario from this document. To make the list, I looked at every lab I had in my drive, every available LAB FRQ, the equation sheet, and my lab equipment and tried to come up with as many possible scenarios that were relevant to the course. As of writing, I have 29 scenarios for units 1-4 and in the next few weeks, I hope to have units 5-8 added.
For a given scenario, my students answer the 8 questions, which have them practice writing procedures, understanding what they would need to do to linearize, and think about their potential error (which I emphasize over and over: in science, error =/= mistake, it’s the assumptions we’re making that oversimplify our system).
My typical implementation is to have them write independently on whiteboards for a few minutes, then work with a partner or group to further refine their ideas, then we build up a final answer as a class. This can be done in as little as 15 minutes or it could take upwards of 45, depending on how you want to roll. I’ve committed the first 15 minutes of class to drills (like my flashcards) because I need need need students to have fluency with the basics and they weren’t building it on their own, so I see this as a worthwhile investment of limited class time.
I came up with this over winter break, so better late than never, but next year I’m going to start using this as early in the year as we can, so hopefully by the end of the year we will have tested out every single scenario.
Final Thoughts
We all know the AP Exam and curriculum is not perfect and if we’re tasked with teaching AP we are beholden to the almighty College Board, but I do think there are some good skills that are emphasized by this particular FRQ. Being able to take a physical situation, manipulate it, analyze it, and get new information is pretty foundational for future scientific practice. I hope my attempts to process and understand this question benefit you and your students.
If you like the idea of my experimental scenario drill, please take a copy for yourself and if you tweak it, expand on it, improve it, or think it’s a waste of time, please let me know in the comments, on Instagram, or via email at ceesquaredscience@gmail.com.
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