FRQ 3 – Experimental Design

Overview

AP Physics 2 FRQ 3 is the Experimental Design and Analysis question (often called the LAB question). It's worth 10 of the 40 free-response points, comes with a suggested time of 25-30 minutes, and asks you to design an experiment, then analyze real data from a related experiment. The free-response section as a whole has 4 questions, lasts 100 minutes, and counts for 50% of your AP Physics 2 score.

The LAB question splits into two halves. In the Design half, you create a procedure that could answer a physics question using equipment found in a typical high school lab, and you describe how to analyze the data your procedure would produce. In the Analysis half, you're handed a data table from a similar (but not identical) experiment, and you plot a graph, draw a best-fit line, and use a feature of that line (usually the slope or intercept) to calculate a physical quantity. It's the one FRQ that tests whether you can think like an experimental physicist, not just solve problems.

How the AP Physics 2 LAB Question Is Scored

FRQ 3 is worth 10 points, and the points split roughly between designing the experiment and analyzing the provided data. There's no single fixed rubric (it varies by year), but scoring guidelines consistently reward the same elements. Here's the typical breakdown:

Two rules from the exam itself shape everything you write. First, your procedure must vary a single parameter and measure how that change affects a single characteristic. Second, your method must be doable in a typical high school lab with realistically obtainable equipment or sensors. Proposing an oscilloscope or a perfectly frictionless track costs you points.

Heads up: starting with the May 2027 exam, the multiple-choice section grows to 42 questions in 85 minutes and the FRQ section shrinks to 95 minutes. There are still 4 FRQs, so your per-question budget tightens slightly.

How to Answer the LAB Question, Step by Step

The biggest trap on FRQ 3 is spending so long writing a beautiful procedure that you rush the graph, which is often the highest-value chunk of points. Budget your 25-30 minutes deliberately.

Read both halves first (3-4 minutes)

Read the entire question, including the data table at the end, before writing anything. The analysis data tells you what kind of relationship the question is building toward. If part (a) asks how to test whether a component is ohmic and part (c) gives you current readings for different resistances, you know graphical linearization is coming, and your design answer can set up that thinking.

Draw the setup (2-3 minutes)

Make the diagram functional, not just pretty. Graders check whether your setup would actually produce the measurements your procedure claims. For a circuit question, that means the voltmeter is in parallel with the component you're measuring and the ammeter is in series with it. Label every component using the symbols given.

Write the procedure (5-6 minutes)

A scoring procedure has three properties.

1. It varies one thing systematically. "Adjust the resistance to five different known values" beats "change the circuit."
2. It measures one thing in response. Say exactly what instrument reads exactly what quantity at each step.
3. It's replicable. Pretend a student in another school has to run your experiment from your words alone. If they'd have to guess, add detail.

Then add an uncertainty step with a reason. "Repeat measurements" alone is usually not enough. "Take three measurements at each setting and average them to reduce random error in the meter readings" connects the step to the error it fixes, which is what the rubric rewards.

State the analysis method (2-3 minutes)

"Analyze the data" earns zero points. You need two specific sentences: what to graph, and how to interpret it. For the ohmic example: "Plot current vs. voltage. A straight line through the origin indicates ohmic behavior, and the slope equals 1/R." Always say which axis gets which quantity and what the slope or intercept means physically.

Build the graph (5-6 minutes)

This is where careless students bleed points. The checklist:

• Label both axes with the quantities AND units.
• Choose a scale so your data fills most of the grid (a graph squished into one corner loses the scaling point).
• Plot every point accurately. Double-check the ones near the edges.
• Draw a best-fit line, not a dot-to-dot connect. And don't force it through the origin unless the data actually supports that.

Sometimes you have to linearize first. The released sample asks you to find a battery's internal resistance $r$ from current measurements at different known resistances $R$. Plotting $I$ vs. $R$ gives a curve, which you can't easily analyze. But Kirchhoff's loop rule gives $\varepsilon = I(R + r)$, which rearranges to $\frac{1}{I} = \frac{1}{\varepsilon}R + \frac{r}{\varepsilon}$. Plot $1/I$ vs. $R$ and you get a straight line whose slope is $1/\varepsilon$ and whose vertical intercept is $r/\varepsilon$. The question even gives you blank columns in the data table for calculated quantities like $1/I$, which is a strong hint that linearization is expected.

Calculate from the line (3-4 minutes)

Use the best-fit line, not individual data points, to find the slope or intercept. In the internal resistance example, dividing the intercept by the slope gives $r = \frac{r/\varepsilon}{1/\varepsilon}$. Pick two well-separated points ON your line for the slope calculation, show the arithmetic, and attach units. Leave 1-2 minutes to verify axis labels and units before moving on.

Worked Example: Testing Whether a Component Is Ohmic

A released LAB question gives you known resistors, an ideal battery, a voltmeter, an ammeter, and connecting wires, and asks you to determine whether an unknown component $R_X$ is ohmic. Here's how the design half plays out (this walkthrough is strategy, not an official key).

An ohmic component obeys Ohm's law, meaning current through it is directly proportional to the voltage across it. So the experiment must vary the voltage across $R_X$ and measure the resulting current.

Diagram: $R_X$ in series with the battery and ammeter, with the voltmeter in parallel across $R_X$ only. Swapping in different known resistors in series with $R_X$ changes how much of the battery's voltage drops across $R_X$, which is how you vary the independent variable with the given equipment.

Procedure (example wording): "Connect the circuit as drawn with one known resistor in series with $R_X$. Record the voltmeter reading across $R_X$ and the ammeter reading. Replace the known resistor with each of the other known resistances, recording $V$ and $I$ each time, for at least five different values. Take each reading three times and average to reduce random error in the meters."

Analysis method: "Plot $I$ on the vertical axis vs. $V$ on the horizontal axis. If the data form a straight line passing through the origin, $R_X$ is ohmic, and the slope equals $1/R_X$. If the graph curves or has a nonzero intercept, $R_X$ is non-ohmic."

Notice the structure: one varied parameter, one measured response, replicable steps, an error-reduction step with a reason, a named graph, and an interpretation criterion. That's the full design rubric in four short paragraphs.

Common Experimental Scenarios to Recognize

Most LAB questions draw from a handful of setups, and recognizing them saves precious minutes. Circuits questions include internal resistance (vary the load, measure current or terminal voltage) and ohmic-behavior tests. Optics questions include focal length from object/image distances and index of refraction from measured angles. Wave questions use standing waves or resonance tubes, where you relate lengths to wavelengths. Thermal questions include calorimetry and gas-law verification, where a good procedure includes "wait for thermal equilibrium before reading the thermometer."

Across all of them, the analysis pattern is the same: find a way to rewrite the governing equation in the form $y = mx + b$, plot the right calculated quantities, and read the physics off the slope and intercept. If you practice that one move (linearization), you can handle a LAB question from any unit.

Common Mistakes

• Varying more than one thing at a time. A procedure that changes both the resistance and the battery isn't a controlled experiment and loses procedure points. Pick one independent variable and hold everything else fixed.
• Writing a procedure no one could replicate. "Set up the circuit and take measurements" is too vague to score. Name the instrument, the quantity it reads, how many values you'll test, and the order of steps.
• Saying "repeat trials" with no reason. The uncertainty point usually requires connecting the step to the error it addresses. Averaging repeated readings reduces random error; it does nothing for a miscalibrated meter (a systematic error).
• Vague analysis plans. "Graph the data and look at it" earns nothing. State the axes, the expected shape, and what the slope or intercept tells you.
• Graph hygiene failures. Missing units on axes, a scale that uses less than half the grid, connecting dots instead of drawing a best-fit line, or forcing the line through the origin. These are free points lost on pure carelessness.
• Calculating slope from data points instead of the line. Use two well-separated points on your best-fit line. Using raw data points (especially adjacent ones) gives a noisy slope and can cost the calculation point.

Practice and Next Steps

The LAB question rewards reps more than any other FRQ, because graph construction and linearization are physical skills you build by doing. Work released experimental design questions from the AP Physics 2 past exams collection, then get instant rubric-style feedback with FRQ practice with scoring. When you draft a procedure, score yourself against the checklist above: one variable, one measurement, replicable, uncertainty step with a reason.

Once FRQ 3 feels comfortable, round out the rest of the section with the guides to FRQ 1, Mathematical Routines and FRQ 4, Qualitative/Quantitative Translation. Then take a full-length AP Physics 2 practice exam under timed conditions and check where you stand with the AP score calculator.