Forces and Free-Body Diagrams
A force is a vector interaction between two objects or systems; a free-body diagram isolates one chosen object or system and shows every external force exerted on it.
What do I need to know for AP Physics 1 Topic 2.2?
Topic 2.2 Forces & Free-Body Diagrams: A force is a vector interaction between two objects or systems; a free-body diagram isolates one chosen object or system and shows every external force exerted on it.
- Identify real interactions, distinguish contact from long-range forces, draw complete force vectors from the center-of-mass representation, and translate the diagram into component equations.
- Practice target: 42 original MCQs and 4 original FRQs for this exact topic.
- Course alignment: current AP Physics 1 framework for the 2026–27 school year and May 2027 exam.
What is Forces and Free-Body Diagrams?
A force is a vector interaction between two objects or systems; a free-body diagram isolates one chosen object or system and shows every external force exerted on it.
Physics terms this topic teaches directly
Use these as working definitions for the lesson. Each definition is paired with a physical situation so the vocabulary is connected to observable motion or interactions rather than memorized in isolation.
Force
A force is a vector interaction exerted by one object on another that can contribute to changing the object’s velocity. Forces are measured in newtons.
Force as an interaction
A force exists because two objects or systems interact; naming a force correctly identifies both the object exerting the force and the object receiving it.
Force vector
A force vector represents both the magnitude and direction of one physical interaction acting on the chosen object or system.
Free-body diagram
A free-body diagram is a representation of one chosen object or system showing every external force acting on it as a vector and excluding forces it exerts on other objects.
External force
An external force is a force exerted on the chosen system by an object outside the system boundary.
Net external force
Net external force is the vector sum of all external forces acting on the chosen system. It is a result of adding forces, not an additional force to draw.
Normal force
Normal force is the contact force a surface exerts perpendicular to itself. Its magnitude is determined by the actual perpendicular dynamics and is not automatically equal to mg.
Understand these ideas here before continuing
You should not have to leave this lesson just to decode its vocabulary. Each concept below is defined here, connected to the physics of this topic, and illustrated with a real situation. A related lesson link is provided only for deeper study.
System
Meaning here: The object or group of objects you isolate for analysis; only forces from outside that boundary appear as external forces on the system FBD.
How this topic uses it: Before drawing an FBD you must state what object or group the diagram represents. The same physical interaction can be external for one system choice and internal for another.
Vector
Meaning here: A quantity with magnitude and direction. Every force on an FBD is drawn as a vector arrow.
How this topic uses it: Forces have magnitude and direction, so an FBD is a vector model rather than a list of force names. Arrow direction and relative size communicate information that later becomes component equations.
Acceleration
Meaning here: The rate at which velocity changes; the net external force points in the same direction as acceleration.
How this topic uses it: The FBD is ultimately used to predict acceleration from the net external force. Acceleration need not point with velocity, which is why a braking car can move forward while the net force points backward.
Vector components
Meaning here: Perpendicular parts used after the FBD to write equations along chosen axes.
How this topic uses it: After whole force vectors are drawn, components let you apply Newton’s second law along convenient perpendicular axes. Components are a calculation tool, not extra forces to add to the FBD.
How Forces and Free-Body Diagrams works
Build the physical model first. The examples below connect the idea to situations you could actually observe before the page turns to equations.
A force is an interaction, so every arrow needs a source
Do not begin a free-body diagram by guessing arrows from the object's motion. Instead, identify what is touching the chosen object and what can interact with it at a distance. A book on a table has Earth pulling downward and the table pushing upward. A sled pulled by a rope can also have tension and friction. Naming the interaction—“Earth on book,” “table on book,” “rope on sled”—prevents invented forces such as a “force of motion.” In this unit, gravity is the long-range force; normal force, friction, and tension are contact interactions.
Draw the whole external forces before resolving components
Represent the chosen object or system by a dot or simple shape and draw each external force as one vector. Do not replace weight with its x and y components on the free-body diagram, and do not combine two distinct same-direction forces into one unnamed arrow. For a crate on an incline, draw weight vertically downward, normal perpendicular to the surface, and any tension or friction in their actual directions. Choose axes afterward—often parallel and perpendicular to the incline—then resolve the whole forces into components for equations.
The diagram becomes a set of signed Newton-second-law equations
Once the free-body diagram is complete, write the net-force equation separately along each chosen axis. If a crate accelerates up a ramp, the signs in ΣF_parallel = ma_parallel come from the axis choice, not from whether a force “helps” or “hurts.” A system cannot exert a net force on itself, so self-forces never belong on its free-body diagram. At the microscopic level, the normal and friction forces used in these macroscopic diagrams arise from electromagnetic interactions between atoms in contacting materials.
Core ideas with real physical examples
Each category keeps the physics statement and the real situation together, so the page does not reduce the topic to abstract definitions.
| Idea/type | What it means | Real example 1 | Real example 2 |
|---|---|---|---|
| Weight / gravitational force | Long-range force exerted by Earth on an object near Earth. | A dropped ball has weight downward even while moving upward. | A book on a desk still has weight downward while at rest. |
| Normal force | Contact force perpendicular to a surface. | A table pushes upward on a resting book. | An incline pushes perpendicular to its tilted surface, not vertically upward. |
| Tension | Pulling interaction transmitted by a taut string, rope, cable, or chain. | A hanging lamp is pulled upward by its cable. | A sled pulled by a rope experiences tension along the rope. |
| Friction | Contact force parallel to the interface that opposes relative slipping or its tendency. | A sliding box experiences kinetic friction opposite the sliding. | Static friction can push a rolling tire forward without the tire slipping. |
| Contact vs long-range interaction | Contact forces require touching surfaces; in AP Physics 1, gravitational force is the long-range interaction used in this unit. | A desk exerts a normal force only while the book is in contact with it. | Earth exerts gravity on a satellite without physical contact. |
| Whole forces on an AP free-body diagram | Draw each external force as its own straight vector from the dot/center-of-mass representation; do not replace the force with x/y components on the FBD. | On an incline, draw the single weight vector vertically downward, then resolve it into components in the equations—not as two separate weight forces on the FBD. | If two different ropes pull right, draw two separate rightward tension arrows side by side rather than overlapping them. |
| A system cannot exert a net force on itself | Only interactions with the environment contribute to the net external force on the chosen system; internal force pairs cancel in the whole-system force sum. | For two carts treated as one system, the spring forces the carts exert on each other are internal and do not create a net external force on the two-cart system. | A person cannot accelerate the person-plus-skateboard system horizontally by pushing only on the skateboard; an external interaction with the ground is needed. |
| Microscopic origin of contact forces | Normal force, friction and tension are macroscopic models of electromagnetic interactions between atoms and molecules at contact. | A table’s normal force on a book comes from microscopic electric repulsion as the surfaces resist interpenetration. | Friction between a tire and road is modeled macroscopically even though its microscopic origin is electromagnetic interaction between the contacting materials. |
| Choose useful axes after the diagram | Axes can be chosen to simplify force components—often parallel to acceleration or parallel/perpendicular to an incline—but the FBD still shows whole forces first. | For a block on an incline, choose x along the slope and y perpendicular to it after drawing weight vertically downward. | For an elevator, a vertical y-axis lets tension and weight enter one signed equation directly. |
College Board focus for Topic 2.2
2.2.A–B · Describe forces as interactions and represent all external forces on a free-body diagram.
Important boundary: Draw whole-force arrows first; resolve components after the FBD. Do not add self-forces or motion arrows.
Equations, variables, SI units and when they apply
| Relationship | Variables / SI units | When to use it | Real example |
|---|---|---|---|
| F⃗net = ΣF⃗ | F: newton (N) | Add all external force vectors on the chosen object/system. | 30 N right and 10 N left produce a 20 N right net horizontal force. |
| Fx = F cosθ, Fy = F sinθ | F components: N; θ: angle | Resolve an angled force when axes are chosen horizontally/vertically. | A rope pulled 30° above horizontal has horizontal and vertical components that affect acceleration and normal force. |
Interactive Forces and Free-Body Diagrams check
Net-force component checker
Enter right, left, up and down force magnitudes to see the signed net components.
Adjust a slider to explore the model.
Three real-situation worked examples
Book on a table
The book is motionless.
Crate pulled on rough floor
A rope pulls right while the crate slides right.
Ball after release
A ball has just left a person’s hand and is traveling upward.
Common AP mistakes—and the correction
| Common mistake | Correct physics | Real-life check |
|---|---|---|
| Drawing velocity or acceleration arrows on the FBD as forces. | Only actual interaction forces belong on an FBD. Motion vectors can be shown separately. | Book on a table: test the statement against this physical situation. |
| Putting a third-law partner on the same object’s FBD. | Partner forces act on different objects and belong on different FBDs. | Crate pulled on rough floor: test the statement against this physical situation. |
| Assuming normal force always equals mg. | N depends on the surface orientation and other vertical/perpendicular forces. | Ball after release: test the statement against this physical situation. |
| Drawing friction automatically opposite velocity. | Friction opposes relative sliding or the tendency to slide at the contact. | Book on a table: test the statement against this physical situation. |
Master this topic beyond a formula list
This checkpoint combines the current AP Physics 1 content boundary with representation, experiment/data reasoning, real situations and cross-topic transfer.
2026–27 mastery checklist
- Describe a force as an interaction between two objects rather than a property an object “has.”
- Draw only external forces acting on the chosen system, with complete vector direction and meaningful labels.
- Distinguish contact forces from long-range gravitational interactions.
- Choose axes strategically and resolve force components only after the physical force diagram is correct.
Must-not-miss relationship
A free-body diagram contains forces, not velocity, acceleration, “ma,” or forces the chosen object exerts on something else.
Two anchor situations
A book on a table has Earth-on-book gravitational force downward and table-on-book normal force upward.
After a ball leaves a hand, there is no continuing “throw force”; in the ideal model only gravity acts.
Diagram / graph / equation / claim
Convert a real scene into an interaction list, then an FBD, then component equations. Reverse the process by reconstructing a plausible scene from an FBD.
How to test the physics
Use a force sensor and hanging/dragged object to compare measured tension with an FBD-based prediction; include uncertainty and calibration considerations.
Draw whole forces first. Components are mathematical parts of a force, not additional physical forces.
Use a correct FBD as the representation bridge into Newton’s laws, friction, springs, circular motion and buoyancy.
Connect the FBD to Topic 2.5 dynamicsGo beyond memorizing the equation
This topic now combines model selection, physical intuition, a worked transfer problem, graph/data analysis, answer verification, and the four current AP free-response reasoning modes.
Choose the model before calculating
Isolate one object or system, list only forces exerted on it by the environment, draw whole-force arrows, then choose axes and resolve components afterward.
Why the physics works
A free-body diagram is an interaction inventory. Omitting a real external force or adding a non-force such as “ma” changes the physical model before any algebra begins.
Solve this before opening the solution
A 10 kg crate is pulled right by 50 N while kinetic friction is 20 N left. Vertical forces balance. Find horizontal acceleration.
Reveal complete reasoning
Solution: ΣF_x=50−20=30 N, so a_x=30/10=3 m/s² right.
Physics check: Acceleration points in the direction of the net force and is smaller than 50/10 because friction opposes the pull.
What should you plot?
For a family of cases with fixed mass, plot acceleration versus net force after constructing a correct FBD for each case.
What does the graph mean?
The slope should be 1/m, but only if every plotted net force came from a complete force inventory.
How this exact topic can appear in each current FRQ mode
A free-body diagram contains forces, not velocity, acceleration, “ma,” or forces the chosen object exerts on something else.
Convert a real scene into an interaction list, then an FBD, then component equations. Reverse the process by reconstructing a plausible scene from an FBD.
Use a force sensor and hanging/dragged object to compare measured tension with an FBD-based prediction; include uncertainty and calibration considerations.
Use a correct FBD as the representation bridge into Newton’s laws, friction, springs, circular motion and buoyancy.
Earn representation credit by labeling each force by interaction, not by motion, and do not draw components in place of the original force unless requested.
Continue with the exact topic question banks
Use the focused banks for skill practice. The five full model papers are reserved for whole-exam simulation.
Topic 2.2 quick questions
What is the first thing I should identify in a Topic 2.2 problem?
Identify real interactions, distinguish contact from long-range forces, draw complete force vectors from the center-of-mass representation, and translate the diagram into component equations.
Should I memorize an equation before understanding the physical model?
No. Start by identifying the system, interactions, directions and representation. Then choose the relationship whose assumptions match the situation.
Where should I practice this exact topic?
Use the direct 42-MCQ and 4-FRQ Topic 2.2 banks on this page. The five-model-paper page is reserved for full-exam simulation.
Is this aligned to the May 2027 AP Physics 1 exam?
Yes. The course content remains based on the Fall 2024 framework for 2026–27; the Fall 2026 update changes the May 2027 exam timing and question count, not this Unit 2 topic sequence.
Where Topic 2.2 appears in AP-style released work
These are original topic summaries and alignment notes, not reproduced College Board question text. One released FRQ can legitimately connect to several topics.
Atwood-style dynamics and force analysis
Strong Newton’s-second-law and free-body-diagram connection.
Mathematical RoutinesAtwood system, free-body diagrams and acceleration
Direct dynamics, system and force-diagram connection.
Mathematical RoutinesMeterstick balance and torque experiment
Applies torque, equilibrium, measurement design and uncertainty.
Experimental Design & AnalysisRecent public FRQs are the safest format reference. Older released questions can still contain excellent physics, but the course and exam changed; use the atlas alignment notes before treating a legacy question as a current-format simulation.