Newton’s Third Law
When object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A.
What do I need to know for AP Physics 1 Topic 2.3?
Topic 2.3 Newton’s Third Law: When object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A.
- Identify interaction pairs, keep paired forces on different objects, understand why internal pairs cancel for a whole system, and use the ideal-string/ideal-pulley tension model.
- 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 Newton’s Third Law?
When object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A.
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.
Newton’s third law
When object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and opposite direction on object A. The two forces act on different objects.
Newton-third-law interaction pair
A third-law pair consists of the two forces produced by the same interaction, one on each interacting object. They never cancel on a one-object free-body diagram because they act on different objects.
Tension
Tension is the pulling force transmitted by a taut string, rope, or cable. In an ideal massless string over ideal supports, the tension magnitude is uniform.
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.
Interaction
Meaning here: A mutual physical influence between two objects; forces arise from interactions rather than from motion itself.
How this topic uses it: Newton’s third law is about one interaction shared by two objects. Thinking in terms of interactions prevents the common mistake of treating the pair as two forces acting on the same object.
System boundary
Meaning here: Whether a Newton-third-law pair is internal or external depends on which objects are included in the chosen system.
How this topic uses it: Whether a third-law pair is internal or external depends on the chosen system. For a system containing both interacting objects, the pair is internal to that larger system.
Force vector
Meaning here: Each member of a third-law pair has the same magnitude and opposite direction, but the forces act on different objects.
How this topic uses it: Each partner force has the same magnitude and opposite direction, but each vector belongs to a different object. That is why the pair does not cancel on a one-object FBD.
Free-body diagram
Meaning here: An FBD contains only forces acting on the one chosen object/system; the partner force usually belongs on the other object’s FBD.
How this topic uses it: Separate FBDs reveal where the two members of a third-law pair belong. A force should appear only on the diagram of the object that experiences it.
How Newton’s Third Law works
Build the physical model first. The examples below connect the idea to situations you could actually observe before the page turns to equations.
Third-law forces are equal and opposite but act on different objects
If object A exerts a force on object B, object B simultaneously exerts an equal-magnitude force in the opposite direction on object A. The pair is one interaction viewed from its two sides. During a truck–car collision, the truck pushes the car and the car pushes the truck with equal force magnitudes. Those two forces do not cancel on the car because only one member of the pair acts on the car; the other acts on the truck.
Equal force does not mean equal acceleration
Newton's third law fixes the force pair, while Newton's second law determines each object's acceleration. In the same truck–car collision, the lighter car can have the larger acceleration because a = F/m. Earth and a falling ball also pull on each other with equal gravitational forces, but Earth's enormous mass makes its acceleration tiny. This distinction is a frequent AP reasoning target: compare forces with the third law, then compare accelerations with the second law.
String tension depends on the string model
For a massless, nonstretching ideal string passing over an ideal pulley, the tension magnitude is the same throughout the string. That lets two connected objects share a single tension variable in many AP problems. A real massive rope, cable, or chain can have different tension at different points because sections of the rope themselves must be accelerated; in AP Physics 1 that variation is treated qualitatively. In this force unit, long-range third-law interaction examples are gravitational rather than electric or magnetic.
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 |
|---|---|---|---|
| Interaction pair | Two forces from one interaction, equal in magnitude and opposite in direction. | A truck pushes a car; the car pushes the truck equally hard in the opposite direction. | Earth pulls a person downward; the person pulls Earth upward with equal gravitational force. |
| Different objects | The two third-law forces never act on the same object. | Table-on-book normal acts on the book; book-on-table normal acts on the table. | Bat-on-ball and ball-on-bat forces act on different bodies. |
| Internal-force cancellation | When both interacting objects are inside one system, each pair cancels in the whole-system force sum. | Two skaters pushing each other do not create a net internal force on the two-skater system. | Spring forces between two carts cancel when the carts are analyzed together. |
| Ideal tension | A massless nonstretching string over an ideal pulley has the same tension magnitude throughout. | Two blocks connected by one ideal string share one tension magnitude. | A massive rope can have different tensions along its length, so the ideal rule no longer applies exactly. |
| Ideal pulley | An ideal pulley has negligible mass and negligible axle friction, so it does not require a tension difference to rotationally accelerate the pulley. | A light ideal pulley redirects a string without changing the ideal-string tension magnitude. | A real massive pulley can require unequal tensions on its two sides, so the simple ideal model may fail. |
| Massive-string boundary | For a string, cable, or chain with nonnegligible mass, AP Physics 1 expects qualitative reasoning about changing tension rather than unrestricted quantitative tension distributions. | A hanging heavy chain has greater tension near its top because upper links support more chain below. | A massive horizontal rope being accelerated can need different tension at different positions along its length. |
College Board focus for Topic 2.3
2.3.A · Describe equal-and-opposite forces exerted by interacting objects.
Important boundary: Ideal massless strings use uniform tension; massive-string tension variation is qualitative.
Equations, variables, SI units and when they apply
| Relationship | Variables / SI units | When to use it | Real example |
|---|---|---|---|
| F⃗A on B = −F⃗B on A | Force: N | Compare the two forces in one interaction. | During a collision, if car A exerts 4000 N east on B, B exerts 4000 N west on A. |
| T1 = T2 (ideal string) | T: tension (N) | Use only with a massless, nonstretching string and ideal pulley model. | In an Atwood-style ideal string, the tension magnitude is the same on both sides of the pulley. |
Interactive Newton’s Third Law check
Third-law pair visualizer
Enter one interaction-force component. The partner force appears automatically with equal magnitude and opposite sign.
Adjust a slider to explore the model.
Three real-situation worked examples
Truck vs small car
The small car accelerates more during a collision.
Person pushes wall
The wall remains stationary.
Two skaters push apart
One skater has twice the mass of the other.
Common AP mistakes—and the correction
| Common mistake | Correct physics | Real-life check |
|---|---|---|
| “Action happens first, reaction later.” | The two forces are simultaneous parts of one interaction. | Truck vs small car: test the statement against this physical situation. |
| “The heavier object exerts the bigger force.” | Mass changes acceleration response, not the equality of the interaction pair. | Person pushes wall: test the statement against this physical situation. |
| “Weight and normal force are a third-law pair.” | They both act on the same object, so they cannot be a third-law pair. | Two skaters push apart: test the statement against this physical situation. |
| “Equal and opposite forces always cancel.” | Only forces acting on the same chosen system can be combined in one net-force sum. | Truck vs small car: 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
- Identify interaction pairs: force of A on B and force of B on A are equal in magnitude and opposite in direction.
- Recognize that the two forces in a third-law pair act on different objects and therefore do not cancel on a one-object FBD.
- Apply the law simultaneously during contact and long-range interactions.
- Distinguish ideal massless-string tension from qualitative tension variation in a massive string.
Must-not-miss relationship
Newton’s third-law pair is F_A→B = −F_B→A. Equal force does not imply equal acceleration because the interacting objects may have different masses.
Two anchor situations
A truck and a small car exert equal-magnitude collision forces on one another even though the car’s acceleration magnitude is larger.
Earth pulls a falling apple downward with the same force magnitude that the apple pulls Earth upward.
Diagram / graph / equation / claim
Draw separate FBDs for both interacting objects and color-match the third-law pair across the two diagrams rather than placing both arrows on one object.
How to test the physics
Use two force sensors hooked together and pull with different motions. Compare simultaneous readings and discuss sensor uncertainty.
Do not pair weight with normal force as a third-law pair; both can act on the same object and originate from different interactions.
Use third-law/internal-force reasoning to explain why internal forces cancel in whole-system momentum and center-of-mass analyses.
Connect interaction pairs to Topic 4.3Go 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
Identify the two interacting objects and write the pair as “force of A on B” and “force of B on A.” Never put both forces of a third-law pair on one object’s FBD.
Why the physics works
The forces are two sides of a single interaction and therefore have equal magnitude and opposite direction, even when the objects have very different masses or accelerations.
Solve this before opening the solution
A truck collides with a small car. During the collision, which vehicle experiences the larger interaction force and which can have the larger acceleration?
Reveal complete reasoning
Solution: The interaction forces are equal in magnitude and opposite in direction. The smaller-mass vehicle can have the larger acceleration because a=F/m.
Physics check: Equal force does not imply equal acceleration; mass controls the acceleration response.
What should you plot?
If force sensors are mounted on two colliding carts, plot F_AonB versus F_BonA at matched times.
What does the graph mean?
The data should have equal magnitudes with opposite signs; a plot of one versus the other should follow a slope near −1.
How this exact topic can appear in each current FRQ mode
Newton’s third-law pair is F_A→B = −F_B→A. Equal force does not imply equal acceleration because the interacting objects may have different masses.
Draw separate FBDs for both interacting objects and color-match the third-law pair across the two diagrams rather than placing both arrows on one object.
Use two force sensors hooked together and pull with different motions. Compare simultaneous readings and discuss sensor uncertainty.
Use third-law/internal-force reasoning to explain why internal forces cancel in whole-system momentum and center-of-mass analyses.
Name both objects in the force labels and explicitly state that the pair acts on different objects.
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 2.3 quick questions
What is the first thing I should identify in a Topic 2.3 problem?
Identify interaction pairs, keep paired forces on different objects, understand why internal pairs cancel for a whole system, and use the ideal-string/ideal-pulley tension model.
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 2.3 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 2.3 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 system, free-body diagrams and acceleration
Direct dynamics, system and force-diagram connection.
Mathematical RoutinesAtwood-style dynamics and force analysis
Strong Newton’s-second-law and free-body-diagram connection.
Mathematical RoutinesRecent 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.