Newton’s Third Law
Lesson notes
Overview
Forces come in equal and opposite pairs: colliding blocks exchange momentum in opposite directions. This topic is rated beginner and is number 9 of 227 in Physics. Core ideas and first intuitions. No prior knowledge needed.
What you are seeing
The panel above shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
When body A pushes on body B, B pushes back on A with a force of equal size and opposite direction: F_AB = −F_BA. The two forces act on different objects, so they never cancel each other in a single free-body diagram. That is how a rocket accelerates by pushing gas backward, and how a swimmer moves by pushing water. In a collision the objects exchange momentum, one gaining exactly what the other loses.
Reading the relationship
The expression "F_AB = −F_BA" is a compact summary of the idea. It uses the symbols F_AB, F_BA; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.
Try this
Start by only moving one slider at a time. Say aloud what you expect, then check.
Common pitfalls
Check units and directions first. Many mistakes come from mixing up a scalar with a vector, or from using a formula outside the conditions where it holds.
How it connects
It builds on “Newton’s Second Law”, the topic before it in Physics. Once you are comfortable, continue with “Inclined Planes”.
Why it matters
Physics explains how the world moves, heats, shines and interacts, and the same principles power technology around you.
Where you might use it
- Revising “Newton’s Third Law” before a class, exam or certification
- Designing vehicles, bridges and devices where motion, forces or waves must be predicted
- Understanding how sensors, lasers and communication links actually work
- Explaining everyday phenomena such as sound, light, heat and orbits
Frequently asked questions
What is Newton’s Third Law?
Forces come in equal and opposite pairs: colliding blocks exchange momentum in opposite directions. It is a beginner topic in Physics.
How do I use the Newton’s Third Law simulation?
The panel at the top shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression F_AB = −F_BA packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Core ideas and first intuitions. No prior knowledge needed. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Newton’s Second Law”, the previous topic in Physics. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.
Quick quiz
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Friction
Increase μ until the block stops sliding: static friction can hold it in place.
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On a flat surface the normal force balances weight exactly; tilt it and the normal force shrinks.