--- title: "Newton's Third Law" description: "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" canonical: https://lightmysky.com/learn/science/newtons-third-law-mt_fbYbe3YSVj source: https://lightmysky.com/learn/science/newtons-third-law-mt_fbYbe3YSVj.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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 Subject: Science · Area: Forces & Motion · Ages 12 to 13 Page: https://lightmysky.com/learn/science/newtons-third-law-mt_fbYbe3YSVj ## Ready when they can - 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 ## Lesson: Newton's Third Law 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). *(drawing: Your 50 N push on the wall has a 50 N partner on you, pointing the opposite way. The bars compare sizes, not directions.)* **Example.** 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. *(drawing: Read these as simultaneous partner forces, not events in order. Each description names who pushes and who receives the push.)* 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. *(drawing: The source and receiver swap. The table's contact force belongs to a different interaction.)* 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. *(drawing: Both receive 12 N. These bars compare accelerations, 4 m/s² and 2 m/s², rather than forces.)* 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. *(drawing: Both forces act on the cart. The text gives opposite directions, so subtract their sizes to find 30 N forward.)* **Recap.** 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.* ## Practice 30 questions on this page, each with its working shown. ## Needs first - [Newton's First & Second Laws](https://lightmysky.com/learn/science/newtons-first-and-second-laws-mt_tXxxCFl32J) ## Opens up - [Momentum and Its Conservation](https://lightmysky.com/learn/science/momentum-and-its-conservation-mt_Nn5_DMbK1_)