Newton's Third Law
Forces come in equal and opposite pairs acting on different objects.
About this topic
Newton's third law says that when one object exerts a force on another, the second exerts an equal and opposite force on the first. The pair always acts on different objects, so they never cancel each other.
The two forces are the same type, act at the same time and have the same size. A swimmer pushes water backward and the water pushes the swimmer forward. A rocket pushes gas down and the gas pushes the rocket up. Walking works because your foot pushes back on the ground and the ground pushes you forward.
Example: when a small car and a large truck collide, the force on each is the same size, yet the car suffers the greater acceleration because its mass is smaller. Standing on the floor, your weight pulls Earth up just as Earth pulls you down. Jet engines, guns that recoil and boats with propellers all use the law.
Misconception: if forces are equal and opposite, how can anything move? Because they act on different objects. To find motion, look only at the forces on one object at a time.
Key ideas
- Action and reaction act on different bodies
- Rockets push gas down and are pushed up
- Pairs have equal magnitude
Quick check
Use cases: where Newton's Third Law helps
- Homework: review the key idea behind Newton's Third Law before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Newton's Third Law without notes.
- Visual learning: play with the interactive lab to see how Newton's Third Law changes when you adjust its inputs.
- Teaching a friend: explain Newton's Third Law in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Newton's Third Law?
Forces come in equal and opposite pairs acting on different objects.
What will I learn in Newton's Third Law?
Action and reaction act on different bodies. Rockets push gas down and are pushed up. Pairs have equal magnitude.
Is Newton's Third Law part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Newton's Third Law lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Newton's Third Law?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.