step 1 of 6
You have used resultant forces to explain one object's motion. Now follow the lesson's swimmer: she pushes the pool wall backward with 50 N.
What force does the wall exert on her?
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Newton's Third Law
State and apply Newton's Third Law: every force has an equal and opposite reaction force acting on a different object, distinguishing action-reaction pairs from balanced forces
What a learner can do afterwards
- States Newton's Third Law correctly, identifies both partner forces and the objects they act on, and explains why equal, opposite forces on different objects are partners only if they belong to the same interaction
- Gives at least two real-world examples (e.g. rocket propulsion, swimmer pushing off a wall)
- Distinguishes a Newton's Third Law pair from balanced forces on the same object
- Explains why a gun recoils when fired using Newton's Third Law
- Uses equal interaction forces and Newton's Second Law to calculate and compare the accelerations of interacting objects
1 · Read
Newton's Third Law links two forces between different objects. If A pushes or pulls B, B exerts an equal force on A in the opposite direction at the same instant. Swap source and receiver to find the partner. Either force may be called action. Equality holds for any masses or motions. Forces can change, so compare partners at the same instant. To find an increase in either force, subtract its earlier size from its later size. Force is measured in newtons (N).
Try it together
A swimmer pushes the pool wall backward with 50 N. The wall pushes her forward with 50 N at the same time. Her push acts on the wall, so it cannot cancel the wall's push on her. The wall is fixed to the pool: forces from its supports can balance her push, leaving it still. When her feet leave the wall, both forces in this contact pair end.
A rocket pushes gas out of its nozzle. Gas pushed down exerts an equal upward force on the rocket, called thrust. Upward acceleration needs thrust to exceed weight and other downward forces. Outside air is not needed because the rocket pushes its own gas. A released balloon also pushes air out and receives the opposite push. Thrust points opposite to the escaping gas, even if the rocket or balloon is already moving the other way.
Balanced forces have zero resultant on one object. Earth's pull down and the table's push up balance on a resting book, but are not Third Law partners. Earth's pull on the book partners the book's pull on Earth. The table's push partners the book's push on the table. Partners belong to one interaction, with source and receiver swapped. Different interactions need not have equal forces. A push from a hand need not match the floor's push on a trolley.
Equal interaction forces can cause different accelerations. Newton's Second Law uses each object's resultant: a = F / m, or F = m × a. With other horizontal forces negligible, 12 N gives a 3 kg skater 4 m/s² and a 6 kg skater 2 m/s². To find one skater's acceleration from the other's, calculate F = m × a for the known skater, then use that equal partner force in a = F / m for the other. From rest, the lighter skater gains more speed in the same time.
During firing, expanding gas pushes the bullet forward and the gun backward. The gun's backward motion is recoil. For the gas-gun interaction, the gas pushes the gun backward and the gun pushes the gas forward. These are equal partner forces. The gas and bullet form a separate force pair.
To find a resultant, include only forces on your chosen object. A cart receives 40 N forward and 10 N backward, giving 40 - 10 = 30 N forward. Its 40 N push on the person acts elsewhere, so exclude it. To calculate acceleration, divide the resultant by mass. For an upward rocket, first find thrust from the equal force on its gas, subtract weight and any other downward forces, then divide by the rocket's mass.
Every push or pull has an equal and opposite partner force acting on a different object. That's Newton's Third Law.
Lesson, questions and flashcards written for this dataset. OpenStax College Physics and University Physics Volume 1 were consulted for physics facts.
Readings and questions written for this dataset. Physics fact references: OpenStax College Physics and University Physics Volume 1. Video references: Khan Academy.
More to read
Two force lists
Imagine writing one list headed swimmer and another headed pool wall. Put each force under the object receiving it. The wall's push belongs on the swimmer's list. The swimmer's push belongs on the wall's list.
Those pushes have equal sizes and opposite directions, but they cannot balance on the swimmer. Her list contains only the wall's member of that pair. To explain the wall staying still, look for other forces on the wall, including forces from its supports.
This way of sorting forces separates two questions. Which forces belong to one interaction? Which forces add together on one object?
What the rocket pushes
An engine can produce thrust without outside air. The rocket has its own gas to push. During the interaction, the gas receives a force toward the nozzle's exit, while the rocket receives the opposite force.
Near Earth, thrust is not the rocket's only force. Gravity still acts downward. An upward-pointing thrust can exist even when it is too small to lift the rocket. Motion depends on the resultant of all forces on the rocket.
Keep the objects clear when discussing recoil too. Expanding gas drives a bullet forward and a gun backward. The partner of gas pushing the gun is the gun pushing the gas.
Checking a proposed pair
Two equal forces pointing opposite ways need not be Third Law partners. A hand pushing a cart and a foot pushing a ball could have matching force sizes. They still belong to different interactions.
Check who exerts each force and who receives it. Partners exchange those roles within one interaction. For Earth's gravitational force on a book, the partner is the book's gravitational force on Earth. For a table's contact force on that book, the partner is the book's contact force on the table.
A book at rest can receive balanced forces from Earth and the table. Their balance describes the book's resultant, not a shared Third Law interaction.
After the air stops escaping
Consider a balloon moving right while its opening points left. Escaping air receives a leftward push from the balloon. The balloon receives a rightward push from that air. Both forces exist during the interaction.
When air stops escaping, that source of thrust ends. It does not follow that the balloon must stop at the same instant. Gravity and air resistance still affect its motion.
The same distinction applies after a swimmer leaves a pool wall. The swimmer-wall contact forces end together, but the swimmer can keep gliding. A force pair describes an interaction while it exists. Later motion depends on the forces still acting.
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