Net force is the vector sum of every force exerted on a system. If the net force is zero, the system is in translational equilibrium and its velocity stays constant (Newton's first law); if it's nonzero, the system accelerates in the direction of the net force (Newton's second law, a = F_net/m).
Net force is what you get when you add up every force acting on a system as vectors, accounting for both magnitude and direction. The CED states it directly in 2.4.A.1, where the net force on a system is the vector sum of all forces exerted on the system. It's not a new force you draw on a free-body diagram. It's the bottom line after all the real forces (gravity, normal, tension, friction, applied) have been tallied.
The net force is the deciding vote on motion. When it equals zero, the system is in translational equilibrium and its velocity doesn't change, which is Newton's first law in action. When it's nonzero, the system accelerates in the exact direction of the net force, with magnitude given by Newton's second law. One subtlety the CED calls out in 2.4.A is that forces can be balanced in one dimension but unbalanced in another, so an object can move at constant velocity horizontally while accelerating vertically. The velocity only changes in the direction of the unbalanced force. That's why you almost always break forces into x and y components before summing.
Net force is the connective tissue of Unit 2 (Force and Translational Dynamics). Learning objective 2.4.A asks you to describe when a system's velocity stays constant, and the answer is always about whether the net force is zero. Topics 2.6 and 2.7 then put net force to work, where you sum gravity, normal force, friction, and tension to find acceleration with Newton's second law. The friction equations in 2.7.A and 2.7.B (|F_f,k| = μ_k F_n and |F_f,s| ≤ μ_s F_n) only matter because friction is one term in a net-force sum.
But net force doesn't stay in Unit 2. In Topics 3.6 and 3.7, the centripetal force in uniform circular motion is just the net force pointing toward the center of the circle. In Unit 8, learning objective 8.3.B defines the buoyant force as a net upward force, the collective result of fluid particles pushing on an object from all sides. If you can do net-force bookkeeping, you've got a tool that works in nearly every unit of the course.
Keep studying AP Physics 1 Unit 8
Newton's First Law and Translational Equilibrium (Unit 2)
Equilibrium is just the special case where the net force equals zero (∑F_i = 0). Constant velocity, including sitting still, is the fingerprint of zero net force. If an FRQ says an object moves at constant speed in a straight line, you can immediately set the force sum to zero.
Centripetal Force (Unit 3)
Centripetal force isn't a new kind of force. It's the name for the net force when it points toward the center of a circle. On a Topic 3.7 free-body diagram, real forces like tension, gravity, or friction add up to play that center-pointing role.
Static and Kinetic Friction (Unit 2)
Friction problems are net-force problems in disguise. Static friction adjusts its value to keep the net force at zero until it maxes out at μ_s F_n, and once sliding starts, kinetic friction becomes a fixed term (μ_k F_n) in your force sum opposing relative motion.
Buoyant Force (Unit 8)
The CED literally defines buoyancy through net force. Per 8.3.B.1, the buoyant force is the net upward force a fluid exerts on an object, the sum of countless particle collisions, with magnitude equal to the weight of displaced fluid (F_b = ρVg). Whether an object floats or sinks comes down to the net force on it.
Net force is one of the most heavily tested ideas in AP Physics 1, and you're expected to do more than define it. Multiple-choice stems ask you to rank net forces from free-body diagrams, identify when the net force is zero from velocity graphs, or pick the diagram consistent with a described motion. On FRQs, net force is the engine behind derivations. The 2019 Long FRQ Q2 had you analyze two connected blocks and reason about how their relative masses affect acceleration, which requires writing net-force equations for each block. The 2022 Short FRQ Q1 combined a spring, a string over a pulley, and a hanging block, so the net force on each object changes as the spring stretches. The 2022 Long FRQ Q2 asked for the net gravitational force on a moon from a planet and another moon, a pure vector-addition problem. In every case the winning move is the same. Draw the free-body diagram with only real forces, choose a coordinate system, sum components, and set the sum equal to ma (or zero for equilibrium).
Centripetal force is not a separate force you add to a free-body diagram, and neither is net force. Centripetal force is just the label for the net force when an object moves in a circle, supplied by real forces like tension, gravity, normal force, or friction. If you draw an arrow labeled 'F_c' or 'F_net' alongside tension and gravity on an FRQ diagram, you lose credit. Only real forces with identifiable sources go on the diagram; net force is the result of adding them.
Net force is the vector sum of all forces exerted on a system, so direction matters as much as magnitude.
Zero net force means translational equilibrium, and by Newton's first law the system's velocity stays constant (which includes staying at rest).
A nonzero net force causes acceleration in the direction of that net force, with magnitude given by Newton's second law.
Forces can be balanced in one dimension and unbalanced in another, and velocity only changes in the direction of the unbalanced force.
Never draw 'net force' or 'centripetal force' as its own arrow on a free-body diagram; only real forces like tension, gravity, normal, and friction belong there.
Net force shows up across units, since centripetal force in Unit 3 and buoyant force in Unit 8 are both defined as net forces.
Net force is the vector sum of all forces exerted on a system (CED 2.4.A.1). If it's zero, velocity stays constant; if it's nonzero, the system accelerates in the net force's direction according to Newton's second law.
It can be. An object moving at constant velocity has zero net force, which is exactly what Newton's first law says. Net force tracks changes in velocity, not velocity itself. Only an accelerating object has a nonzero net force.
Centripetal force is just the net force in the special case of circular motion, when it points toward the center of the circle. It's always supplied by real forces like tension, gravity, or friction, never drawn as its own arrow on a free-body diagram.
No. Free-body diagrams show only the individual real forces acting on the object, like gravity, normal force, tension, and friction. Adding an extra 'F_net' arrow is a classic FRQ point-loser because net force is the result of summing those forces, not a force itself.
Pick a coordinate system, break each force into x and y components, and sum each direction separately. Remember the CED's point that forces can be balanced in one dimension but unbalanced in another, like a block sliding horizontally while vertical forces cancel.