Physics 1 · Unit 2: Dynamics · 14 min read · Updated 2026-05-11
System Models — AP Physics 1
AP Physics 1 · Unit 2: Dynamics · 14 min read
1. What is a System Model?★☆☆☆☆⏱ 3 min
A system model is a problem-solving strategy that involves grouping selected objects together for analysis, rather than analyzing each object individually. You mark a system boundary with a dashed line: any object inside the line is part of the system, everything outside is called the surroundings.
This method simplifies complex multi-object problems by eliminating internal forces that cancel out via Newton's third law, leaving only external forces to calculate. The AP Physics 1 exam tests your ability to choose the optimal system boundary to reduce algebra and avoid errors. System models make up a core part of Unit 2 Dynamics, which accounts for 12-18% of your total exam score.
2. Internal vs. External Forces★★☆☆☆⏱ 4 min
For any system, Newton's second law can be written in terms of external forces only:
\sum \vec{F}_{\text{ext}} = M\vec{a}_{cm}
Where $M$ is the total mass of the system, and $\vec{a}_{cm}$ is the acceleration of the system's center of mass. For connected rigid objects moving together, $\vec{a}_{cm}$ equals the acceleration of every object in the system.
3. System Selection Strategy for Internal Force Calculations★★★☆☆⏱ 4 min
You can choose any system boundary you want for a problem; there is no technically wrong choice, but some choices are far simpler than others. The standard strategy for connected-object problems (where you need to find an internal force like tension or normal force) is:
First select the combined system of all connected objects (all moving with the same acceleration) to find the acceleration of the whole system, ignoring all internal forces.
Then select a smaller subsystem (usually one of the individual objects) to solve for the internal force of interest, because the internal force for the combined system becomes an external force for the smaller subsystem.
This strategy eliminates the need to solve simultaneous equations, reducing the chance of algebra and sign errors. It works for any connected objects moving with the same acceleration, including blocks connected by strings, stacked blocks, Atwood machines, and multiple trailers pulled by a truck.
4. Open vs. Closed Systems★★☆☆☆⏱ 3 min
For AP Physics 1, almost all dynamics problems use closed systems, because the standard form of Newton's second law for systems applies directly to closed systems, with no extra terms for mass flow. When mass does enter or leave an object, you can almost always expand your system boundary to include all mass involved in the process to make the system closed, simplifying calculations. AP Physics 1 only tests conceptual understanding of open vs closed systems, not complex mass flow calculations.
5. AP Style Concept Check★★★☆☆⏱ 4 min
Common Pitfalls
Why: Students often remember tension as a pulling force, so they incorrectly add it to the net external force sum.
Why: Students assume all objects in a system share the same acceleration, which is only true for rigidly connected moving objects.
Why: Students default to closing the system around the obvious object (the cart) without checking for mass flow.
Why: Students pick the pulled object first out of habit, leading to incorrect sign when rearranging equations.
Why: Students get focused on horizontal forces and ignore vertical forces, which are needed to calculate friction.