Newton’s third law of motion states that for every action, there is an equal and opposite reaction. This fundamental principle, laid out by Sir Isaac Newton, explains that forces always occur in pairs. If one object exerts a force on a second object, the second object exerts a force of equal magnitude and in the opposite direction on the first.
🚀 The Action-Reaction Principle
The core idea of Newton’s third law is that forces result from interactions between objects. You can’t have a single, isolated force. When you push on a wall, the wall simultaneously pushes back on you with the same amount of force. This pair of forces is often called an “action-reaction” pair.
It’s a common misconception that the reaction force happens after the action force. In reality, these forces are simultaneous.
⚖️ The Formula
The mathematical representation of Newton’s third law is straightforward:
Where:
is the force exerted by object A on object B.
is the force exerted by object B on object A.
The negative sign indicates that the forces are in opposite directions. The magnitudes of the two forces are equal.
💡 Everyday Examples
You can observe Newton’s third law in action all around you:
- Walking: When you walk, your feet push backward on the ground. In reaction, the ground pushes forward on your feet, propelling you forward.
- Rocket Propulsion: A rocket expels hot gases downward (action). The gases, in turn, push the rocket upward (reaction), launching it into space.
- Swimming: A swimmer pushes the water backward with their hands and feet. The water then pushes the swimmer forward.
- A Bouncing Ball: When a ball hits the ground, it exerts a downward force. The ground exerts an equal and opposite upward force, causing the ball to bounce back up.
Even though the forces in an action-reaction pair are equal and opposite, the effect on the two interacting objects can be very different. This is because the acceleration of an object depends on its mass, as described by Newton’s second law ($F = ma$). For instance, when a tiny fly hits the windshield of a massive bus, the force on both is the same. However, the fly experiences a much greater acceleration due to its small mass, while the bus’s motion is barely affected.
The misconception
It’s a common misconception that the reaction force happens after the action force. In reality, these forces are simultaneous. The confusion comes from the words “action” and “reaction,” which make it sound like a sequence of events. But in physics, a force is not an event; it’s an interaction. You can’t have a one-sided interaction.
Think of it like a handshake. When two people shake hands, their hands press against each other at the exact same time. It’s impossible for one person to exert a force on the other’s hand without the other’s hand simultaneously exerting a force back. You wouldn’t say Person A’s “action” of shaking happens first, and then Person B’s “reaction” follows. The handshake itself is the mutual, simultaneous pressing of both hands. That single event is the action-reaction pair.
When you press your finger on a table, the atoms in your finger push on the table’s atoms, and at the exact same instant, the table’s atoms push back. The feeling of pressure in your fingertip is the table’s “reaction” force happening in real-time.
The Earth’s gravity pulls on the Moon (action), and the Moon’s gravity pulls on the Earth (reaction). Both pulls are happening continuously and simultaneously.
So, it’s best to think of “action” and “reaction” as just labels for the two sides of a single, instantaneous interaction. You could just as easily call them Force A and Force B.