Skip to main content
Category: Laws
Type: Classical Mechanics Law
Origin: Physics, 1687, Isaac Newton
Also known as: Action and Reaction Law, Newton’s Third Law of Motion
Quick Answer — Newton’s Third Law states that when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first. This principle explains everything from how rockets propel themselves to why walking is possible.

​
What is Newton’s Third Law?

Newton’s Third Law states that for every action, there is an equal and opposite reaction. When object A exerts a force on object B, object B simultaneously exerts an equal force in the opposite direction on object A.
“To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.”
This law is counterintuitive because the forces in an action-reaction pair act on different objects. When you push against a wall, the wall pushes back on you with equal force—but since the wall doesn’t move, it seems like your force “disappears.” The key insight is that action and reaction forces always act on different bodies, which is why they don’t cancel out in terms of motion.

​
Newton’s Third Law in 3 Depths

  • Beginner: When you push something, it pushes back. When you walk, your foot pushes backward on the ground, and the ground pushes forward on your foot. You don’t push the ground away—you push off it.
  • Practitioner: In engineering, every system that propels itself (rockets, jets, boats, cars) operates by expelling mass in one direction to generate force in the opposite. The thrust equals the mass flow rate times exhaust velocity.
  • Advanced: The law has fundamental implications for conservation of momentum and energy. In particle physics, even “contact forces” are understood as electromagnetic interactions between atoms—action-reaction at the quantum level.

​
Origin

Sir Isaac Newton (1643-1727) published the third law in his landmark work Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) in 1687. This work, often simply called the Principia, laid the foundation for classical mechanics. The third law emerged from Newton’s systematic analysis of motion. Building on earlier work by Galileo and others, Newton formalized three laws that together describe how objects move under the influence of forces. The third law was particularly revolutionary because it introduced the concept of forces as interactions between pairs of objects, not isolated actions. While the law seems obvious today, it represented a profound shift in thinking. Before Newton, scientists often viewed forces as something an object “had” rather than something exchanged between objects. The action-reaction framework became essential for understanding everything from celestial mechanics to engineering.

​
Key Points

1

Action-reaction pairs

Forces always come in pairs. If object A exerts a force on object B, then object B exerts an equal and opposite force on object A. These two forces form an action-reaction pair.
2

Different objects

The action and reaction forces act on different objects. This is why they don’t cancel out in terms of motion—one force affects object A, the other affects object B.
3

Simultaneity

The forces occur simultaneously. There is no “leading” force—the action and reaction happen at exactly the same instant.
4

Conservation foundation

Newton’s Third Law is fundamental to the conservation of momentum. Without it, momentum could be created or destroyed in collisions.

​
Applications

Rocket Propulsion

Rockets expel gas downward at high speed; the equal and opposite reaction pushes the rocket upward. No air is required—rockets work in the vacuum of space.

Walking and Running

When walking, you push backward against the ground. The ground pushes forward on your foot, propelling you forward. Friction is essential.

Swimming

Swimmers push water backward with their arms and legs. The water pushes back, moving the swimmer forward. This is why swimming in a vacuum would be impossible.

Recoil in Firearms

When a bullet fires forward, the gun recoils backward with equal force. Heavier guns experience less recoil for the same bullet mass.

​
Case Study

​
The Development of the Space Shuttle

The Space Shuttle program provides a dramatic example of Newton’s Third Law in engineering. Each shuttle launch demonstrated the law on a massive scale. The Space Shuttle’s three main engines burned liquid hydrogen and liquid oxygen, expelling approximately 1.6 million pounds of exhaust gases downward at over 6,000 miles per hour every second. According to Newton’s Third Law, the downward force on the exhaust gases produced an equal and opposite upward force on the spacecraft—this is thrust. The two solid rocket boosters added another 2.6 million pounds of thrust each, further demonstrating action-reaction. Together, the system produced over 7 million pounds of thrust at liftoff, accelerating the shuttle from zero to orbital velocity (about 17,500 mph) in just over eight minutes. Interestingly, engineers had to account for more than just the thrust. As the shuttle burned fuel, its mass decreased, requiring careful calculations of how the changing mass would affect acceleration. The simple action-reaction principle, applied rigorously, enabled humanity’s journey into space.

​
Boundaries and Failure Modes

Newton’s Third Law has important considerations:
  1. Not all apparent forces are equal-and-opposite: The law applies to forces arising from direct interaction. Centripetal forces, for example, arise from different physical origins and don’t form action-reaction pairs in the standard sense.
  2. Idealization in contact: Perfect contact is assumed. In real materials, deformations, energy losses, and non-instantaneous force transmission can complicate the simple picture.
  3. Electromagnetic forces at a distance: Modern physics recognizes that electromagnetic forces between objects don’t necessarily manifest as simple push-pull at the macroscopic level.
  4. Relativistic effects: At velocities approaching the speed of light, the simple formulation requires modification. The law still holds but needs relativistic corrections.

​
Common Misconceptions

The forces in an action-reaction pair act on different objects, so they don’t cancel. If they acted on the same object, they could cancel—but then it wouldn’t be an action-reaction pair.
Newton’s Third Law applies to all forces, including gravity and electromagnetism. Objects can exert forces on each other without direct contact.
The forces are simultaneous—there is no “first” action. They occur at exactly the same instant, forming a single interaction.

Conservation of Momentum

The direct consequence of Newton’s Third Law—momentum is conserved in all interactions because action and reaction forces transfer momentum between objects.

Newton's First Law

The law of inertia: an object remains at rest or in motion unless acted upon by a force. Together with the third law, explains how forces cause motion changes.

Newton's Second Law

F = ma (force equals mass times acceleration). This law quantifies how the net force on an object produces acceleration.

​
One-Line Takeaway

For every force you exert on the world, the world exerts an equal force back. This simple symmetry underlies all motion, from walking to rocketry.