SP 3 shows up whenever a prompt asks you to design an experiment, state what a physics law predicts, or explain why something happens. Subskill 3.A is experimental design: you identify the independent and dependent variables, describe a procedure that controls other variables, and specify what data to collect and how to analyze it. Subskill 3.B is applying a law or model to make a specific, testable claim. Subskill 3.C is justification: you connect your claim to a physics principle with explicit logical steps, not just a restatement of the claim.
- Independent variable: The quantity you deliberately change in an experiment; must be identified and isolated from other variables.
- Dependent variable: The quantity you measure in response to changes in the independent variable; your data collection method must target this quantity.
- Claim (3.B): A specific statement about what will happen, grounded in a named physics law or model, not a vague prediction.
- Justification (3.C): An explanation that names the relevant physics principle, connects it to the specific situation, and explains the logical chain from principle to prediction.
A prompt asks you to justify why a cart moving on a frictionless track at constant velocity has zero net force. Can you name Newton's first law, state what it says, and connect it explicitly to the cart's situation in two or three sentences?
| Subskill | What a weak response looks like | What a strong response looks like |
|---|
| 3.A Design | "Measure the force and the acceleration" | "Apply a known net force using a hanging mass, measure acceleration with a motion sensor, repeat for five different masses while keeping the cart mass constant" |
| 3.B Claim | "The object will slow down" | "By Newton's second law, a net force directed opposite to velocity will produce a negative acceleration, so the object will decelerate at a rate of F/m" |
| 3.C Justify | "Because energy is conserved" | "By conservation of mechanical energy, the loss in gravitational PE equals the gain in KE, so (1/2)mv^2 = mgh, giving v = sqrt(2gh)" |