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.
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?