Everything you need to master Unit 2 — free-body diagrams, Newton's three laws, gravity, normal force, friction, springs, and circular motion. The biggest single unit on the exam.
18–23% of the AP exam
7 study resources
College Board aligned
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Seven free resources for Unit 2 — pick the one that fits how you learn.
Unit 2 covers Force and Translational Dynamics — the rules that explain WHY objects move the way they do. This is where you meet Newton's three laws, the most important ideas in classical physics. You'll learn to identify all the forces on an object using a free-body diagram, then use F = ma to find acceleration.
The College Board breaks Unit 2 into 9 topics: (2.1) Systems and Center of Mass, (2.2) Forces and Free-Body Diagrams, (2.3) Newton's Third Law and Tension, (2.4) Newton's First Law and Equilibrium, (2.5) Newton's Second Law, (2.6) Gravitational Force and Weight, (2.7) Kinetic and Static Friction, (2.8) Spring Forces (Hooke's Law), and (2.9) Circular Motion.
Unit 2 is the heaviest unit on the exam at 18–23%. Every other unit builds on these ideas: energy and momentum problems still need free-body diagrams; rotational dynamics is just Newton's laws applied to spinning objects. Spend extra time here — it pays off everywhere.
Key terms preview
A taste of what you'll find in The Essentials and Flashcards.
Newton's Second Law
F_net = ma. The net force on a system equals its mass times its acceleration. The most important equation in all of mechanics.
Free-Body Diagram
A sketch showing every force acting ON an object, drawn as arrows from a single dot representing the center of mass.
Normal Force
The push a surface exerts perpendicular to itself, on whatever's touching it. Not always equal to weight!
Newton's Third Law
Every force has an equal-and-opposite reaction force, acting on the OTHER object. Forces always come in pairs.
Coefficient of Friction
A number (μ) describing how "sticky" two surfaces are. Kinetic friction = μₖ × Normal force.
Centripetal Acceleration
a_c = v²/r. The acceleration of anything moving in a circle, always pointing toward the center.
Newton's second law (F_net = ma) connects forces to motion. If the forces on an object are balanced, it moves at constant velocity (could be zero). If unbalanced, it accelerates in the direction of the net force. The whole unit is essentially applications of this one idea.
2. Forces always come in pairs (Newton's third law)
If A pushes B, then B pushes A with equal force in the opposite direction. The two forces act on different objects, so they don't cancel out. This is why you can push off the ground when you jump — and why internal forces never change a system's overall motion.
3. Circular motion is just Newton's second law pointed inward
Anything moving in a circle is accelerating toward the center (centripetal acceleration = v²/r). That means there must be a net force pointing inward. "Centripetal force" isn't a new kind of force — it's the role played by whatever existing force is doing the inward-pointing work (tension, gravity, friction, normal force).