A free-body diagram (FBD) is a sketch of a single object with labeled arrows representing every external force acting on it, used in AP Physics 2 to find the net force on things like gas pistons, submerged objects, and charged particles.
A free-body diagram is the simplest possible picture of a physics problem. You replace the object with a dot (or a simple box), then draw one labeled arrow for each external force acting on that object. Gravity, normal force, tension, friction, buoyant force, forces from gas pressure, electric force on a charge. Each arrow points in the direction the force acts, and longer arrows mean bigger forces.
The whole point is isolation. You ignore everything the object does to other things and everything happening elsewhere, and you ask one question: what pushes or pulls on this object right now? Once the diagram is drawn, Newton's second law turns it into math. Add the arrows, get the net force, and you know whether the object accelerates or sits in equilibrium. In AP Physics 2, the classic Topic 2.4 example is a piston trapping a gas. The gas pushes up with pressure times area, the atmosphere pushes down, gravity pulls down, and if the piston is in equilibrium those forces balance. That one diagram is how you connect mechanics to thermodynamics.
Free-body diagrams get their own billing in Topic 2.4 (Thermodynamics and Free-Body Diagrams) because AP Physics 2 expects you to carry this Physics 1 skill into new territory. The classic move is analyzing a movable piston on a container of gas. Drawing the FBD of the piston lets you relate the gas pressure inside to atmospheric pressure outside plus the piston's weight, which is often step one of an entire thermodynamics problem. The same skill shows up in fluids (buoyancy and pressure forces) and electrostatics (charged objects in fields). On the exam, a correct FBD is frequently worth explicit points by itself, and an incorrect one can sink every calculation that follows. It's one of the highest-leverage habits in the course. For the full piston-and-pressure treatment, head to the Topic 2.4 study guide.
Keep studying AP Physics 2 Unit 2
Net Force (every unit)
The FBD is the input and net force is the output. You draw the individual forces, then add them as vectors to get the net force, which tells you the acceleration through Newton's second law. The net force itself never gets its own arrow on the diagram.
Buoyant Force (Unit 1)
Fluids problems live and die by FBDs. A floating or submerged object gets a gravity arrow down and a buoyant force arrow up, and comparing their lengths tells you whether it sinks, floats, or hovers. The 2017 fluids FRQ about water flowing through a narrowing pipe is exactly the kind of question where force reasoning on a fluid element earns points.
Gas Pressure on a Piston (Topic 2.4, Unit 2)
Pressure isn't a force, but pressure times area is. The FBD of a piston has an upward force PA from the gas below, a downward force from the atmosphere above, and weight. Setting them equal in equilibrium gives you the gas pressure, which then feeds into the ideal gas law.
Tension and Frictional Forces (Physics 1 review)
Tension and friction are the bread-and-butter FBD forces from Physics 1, and AP Physics 2 assumes you can still draw them. A submerged object tethered to the bottom of a tank, for example, needs gravity, buoyancy, and tension all on one diagram.
Free-body diagrams show up two ways. First, FRQs often explicitly say "draw a free-body diagram" with its own point value, and graders check that every force is present, correctly labeled, starts on the object, and that nothing extra appears (no velocity arrows, no "ma" arrow, no net force arrow). Relative lengths matter too. If the object is in equilibrium, balanced forces should look balanced. Second, FBDs are the hidden first step in quantitative questions. Piston problems in thermodynamics, buoyancy problems in fluids (like the 2017 short FRQ on water flowing through a pipe that changes diameter and elevation), and charged-particle problems in electrostatics all start with identifying the forces before any equation makes sense. In MCQs, expect to pick the correct diagram from four options, where the wrong answers include a missing force, an extra invented force, or arrows with the wrong relative sizes.
A free-body diagram shows the individual forces acting on an object. The net force is the vector sum of those forces, and it does NOT get drawn as a separate arrow on the FBD. Drawing a "net force" or "motion" arrow alongside the real forces is one of the most common ways to lose the FBD point. Find the net force after the diagram, not on it.
A free-body diagram shows only the external forces acting ON one object, drawn as labeled arrows on a dot or box.
Never draw velocity, acceleration, 'ma', or net force as arrows on an FBD; those are results, not forces.
In Topic 2.4, the FBD of a piston connects mechanics to thermodynamics: gas pressure times area pushes one way, atmosphere and weight push the other.
Arrow lengths should match relative force magnitudes, so an object in equilibrium must have balanced-looking arrows.
Pressure alone is not a force; on an FBD you draw the force F = PA that the pressure produces on a surface.
On FRQs, a complete and correct FBD often earns explicit points and sets up every calculation that follows.
It's a sketch showing one object (usually as a dot or box) with a labeled arrow for each external force acting on it, like gravity, buoyant force, tension, or the force from gas pressure. It's the standard first step for applying Newton's second law in Topic 2.4 and beyond.
No. The FBD shows only the individual forces; the net force is what you calculate by adding those arrows as vectors afterward. Including a net force or motion arrow on the diagram can cost you the FBD point on an FRQ.
Because of pistons. Topic 2.4 uses an FBD of a movable piston to relate the gas pressure inside a container to atmospheric pressure and the piston's weight. That force balance is how you find the gas pressure before using the ideal gas law.
Not by itself. Pressure is force per unit area, so on an FBD you draw the force the pressure creates, F = PA, acting perpendicular to the surface. A piston's diagram has PA from the gas on one side and atmospheric pressure times area on the other.
The skill is identical, but the cast of forces grows. Physics 1 leans on gravity, normal force, tension, and friction, while Physics 2 adds buoyant force, pressure forces from fluids and gases, and electric forces on charges. Same diagram, new arrows.