AP Physics C: Mechanics Practice 3: Scientific Questioning and Argumentation is the science practice where you design experiments, apply physics laws to make claims, and back those claims with evidence. In short, you act like a working scientist: you build a procedure that answers a question, pick the right principle or model, and then show why your answer holds up.

This practice has three subskills:

3.A: Create experimental procedures appropriate for a given scientific question.
3.B: Apply an appropriate law, definition, theoretical relationship, or model to make a claim.
3.C: Justify or support a claim using evidence from experimental data, physical representations, or physical principles or laws.

These show up across every unit, from kinematics to oscillations, and they appear on both the multiple-choice and free-response sections.

more resources to help you study

cheatsheets score calculator

What Practice 3: Scientific Questioning and Argumentation Means

Practice 3 is about reasoning, not just calculating. Other practices ask you to draw representations (Practice 1) or run math routines (Practice 2). Practice 3 asks you to connect the dots between a question, a method, a physical principle, and a conclusion.

Think of it in three moves:

Design a way to collect the data you need (3.A).
Choose the correct physics to state what should happen (3.B).
Defend your statement with specific evidence (3.C).

A claim without a reason is incomplete in this practice. You always need to tie the claim to data, a representation, or a law.

What This Practice Requires

Each subskill has a specific job.

3.A: Create experimental procedures

Identify the variables to measure and which to hold constant.
Choose equipment that can actually measure those quantities.
Lay out steps clear enough that someone else could repeat them.
Make sure the procedure answers the stated scientific question.
This subskill appears on the FRQ section (about 30 to 35% of FR weighting for the practice group), not on multiple choice.

3.B: Apply a law, definition, relationship, or model to make a claim

Pick the right principle, such as conservation of energy, Newton's second law, or conservation of angular momentum.
Use it to predict what happens or to state a relationship.
Appears on both MCQ (about 15 to 25%) and FRQ (about 30 to 35%).

3.C: Justify or support a claim using evidence

Point to specific numbers, graph features, diagrams, or principles.
Explain why that evidence supports the claim and not an alternative.
Appears on both MCQ (about 5 to 10%) and FRQ (about 30 to 35%).

These percentages describe how the practice is weighted, based on the course framework.

Skills You Need for This Practice

You will lean on tools from across the course:

Identify variables and controls. Know what changes, what stays fixed, and what you measure.
Match the principle to the situation. Decide whether energy methods, force methods, or momentum methods fit best.
Read representations. Pull meaning from graphs, free-body diagrams, energy bar charts, and data tables.
Compare scenarios. Reason about how a quantity changes when you double mass, halve radius, or raise a ramp.
Write tight reasoning. State a claim, name the evidence, and connect them in one clear chain.

A useful habit: when you make a claim, immediately ask "which law or which data point proves this?"

How It Shows Up on the AP Exam

The exam has 40 multiple-choice questions and 4 free-response questions. Practice 3 appears in both sections.

On multiple choice:

3.B questions ask you to apply a principle and identify the correct claim or prediction.
3.C questions give you data or a representation and ask which conclusion is correctly justified. The right answer usually pairs a correct claim with correct reasoning, so watch for choices that have the right claim but the wrong reason.

On free response:

Question 3 is Experimental Design and Analysis, which is the natural home for 3.A. You may describe a procedure, then analyze the resulting data.
3.B and 3.C appear across multiple FRQs when you make and defend claims.

Practical tip: on FRQ reasoning, write in a claim-evidence-reasoning order so a reader can follow your logic without guessing.

Examples Across the Course

These examples are styled after sample items and show the practice in different units.

Unit 2, Friction (3.C). Two blocks of different mass, area, and coefficient of static friction sit on a board that is slowly tilted. The claim is that the block with the smaller coefficient of static friction slides first, because the angle at which sliding begins depends on the coefficient, not on mass or area. The evidence is the relationship that sliding starts when tan of the tilt angle equals the coefficient of static friction.

Unit 3, Energy (3.B). A student lifts a ball straight up at constant speed. Using the work-energy and conservation framework, the claim is that kinetic energy stays the same while gravitational potential energy increases, so mechanical energy of the ball-Earth system increases. An energy bar diagram supports this directly.

Unit 4, Momentum (3.A). Design an experiment to test conservation of linear momentum in a collision. You would measure each cart's mass and velocity before and after the collision, hold the track level and low-friction, and compare total momentum before and after. The procedure must answer the question of whether total momentum is conserved.

Unit 5, Rotational Dynamics (3.C). Two blocks hang from a string over a wheel that can rotate. The claim is that the two string tensions are unequal, because an unbalanced torque is needed to give the wheel angular acceleration. The supporting principle is Newton's second law in rotational form.

Unit 7, Oscillations (3.B). A block hangs from an ideal spring and oscillates. Using the model of simple harmonic motion, the claim is that acceleration is proportional to displacement and points opposite to it. A graph of acceleration versus time mirrors the position graph inverted, which supports the claim.

How to Practice Practice 3: Scientific Questioning and Argumentation

Try these habits while you study:

Turn every problem into a claim. After you solve, state your result as a sentence and name the law behind it.
Write quick lab plans. Pick a relationship from any unit and sketch a procedure to test it: variables, equipment, steps, and what you measure.
Drill MCQ reasoning. On 3.C questions, eliminate answers that give a correct claim with a flawed reason.
Practice with representations. Take a graph or bar chart and write what claim it supports and why.
Do FRQ 3 prompts. Experimental Design and Analysis questions hit 3.A directly and give you data to defend a claim.

When designing a procedure, check that someone else could repeat it and that it actually answers the question asked.

Common Mistakes

Stating a claim with no evidence. "Block 1 slides first" earns little without the reason tied to the coefficient of static friction.
Choosing the wrong principle. Reaching for force methods when energy or momentum methods are simpler, or applying conservation when an external force or torque is acting.
Confusing correlation with cause. On 3.C items, picking a choice with a true statement that does not actually justify the claim.
Vague procedures. Forgetting to list controlled variables or to specify what you measure and how.
Ignoring system definitions. Claims about conservation depend on whether external forces or torques act on the chosen system.

Quick Review

Practice 3 has three parts: design procedures (3.A), apply a law or model to make a claim (3.B), and justify a claim with evidence (3.C).
3.A lives on the free-response section, especially Question 3. 3.B and 3.C appear on both MCQ and FRQ.
Every claim needs a reason: cite a data point, a representation, or a physical law.
On 3.C multiple choice, match the correct claim with the correct reasoning.
These skills cut across all seven units, from friction and energy to rotation and oscillations.