The Principle of action and reaction It is the third of the laws of motion formulated by Isaac Newton and one of the fundamental principles of modern physical understanding. This principle states that every body A that exerts a force on a body B experiences a reaction of equal intensity but in the opposite direction. For example: jump, paddle, walk, shoot.
The original formulation of the English scientist was as follows:
“With every action an equal and opposite reaction always occurs: it means that the mutual actions of two bodies are always equal and directed in the opposite direction.“
The classic example to illustrate this principle is that when pushing a wall, we apply a certain amount of force on it and it on us an equal but in the opposite direction. This means that all forces are manifested in pairs that are called action and reaction.
The original formulation of this law left out some aspects known today to theoretical physics and did not apply to electromagnetic fields. This law and Newton’s other two laws (the Fundamental law of dynamics and the Inertia law) laid the foundations for the elementary principles of modern physics.
Examples of the principle of action and reaction
- Jump. When we jump, we exert a certain force on the earth with our legs, which does not alter it at all due to its enormous mass. The reaction force, on the other hand, allows us to lift ourselves up into the air.
- Row. The oars are moved by a man in a boat and they push the water with an amount of force that imposes on them; The water reacts by pushing the can in the opposite direction, which results in the advancement on the surface of the liquid.
- Shoot. The force that the explosion of the powder exerts on the projectile, causing it to shoot forward, imposes on the weapon an equal force charge known in the field of weapons as “recoil”.
- Walk. Each step taken consists of a push that we give to the ground backwards, the response of which pushes us forward and that is why we move forward.
- A push. If one person pushes another of the same weight, both will feel the force acting on their bodies, sending them both back some distance.
- Rocket propulsion. The chemical reaction that takes place inside the early phases of space rockets is so violent and explosive that it generates an impulse against the ground, the reaction of which lifts the rocket into the air and, sustained over time, takes it out of the atmosphere. into space.
- The Earth and the Moon. Our planet and its natural satellite attract each other with a force of the same amount but in the opposite direction.
- Hold an object. When taking something in hand, the gravitational attraction exerts a force on our limb and this a similar reaction but in the opposite direction, which keeps the object in the air.
- Bounce a ball. Balls made of elastic materials bounce when thrown against a wall, because the wall gives them a similar reaction but in the opposite direction to the initial force with which we have thrown them.
- Deflate a balloon. When we allow the gases contained in a balloon to escape, they exert a force whose reaction on the balloon pushes it forward, with a speed in the opposite direction to that of the gases leaving the balloon.
- Pull an object. When we pull an object we print a constant force that generates a proportional reaction on our hands, but in the opposite direction.
- Hitting a table. A punch to a surface, such as a table, prints on it an amount of force that is returned, as a reaction, by the table directly towards the fist and in the opposite direction.
- Climbing a crevasse. When climbing a mountain, for example, mountaineers exert a certain force on the walls of a crevice, which is returned by the mountain, allowing them to stay in place and not fall into a void.
- Climb a ladder. The foot is placed on one step and pushes downwards, making the step exert an equal reaction but in the opposite direction and lift the body towards the next one and so on.
- Descend a boat. When we go from a boat to the mainland (a dock, for example), we will notice that by exerting an amount of force on the edge of the boat that propels us forward, the boat will proportionally move away from the dock in reaction.
- Hit a baseball. We print with the bat an amount of force against the ball, which in reaction prints the same force on the wood. Because of this, bats can break while balls are thrown.
- Hammer a nail. The metal head of the hammer transmits the force of the arm to the nail, driving it deeper and deeper into the wood, but it also reacts by pushing the hammer in the opposite direction.
- Push off a wall. Being in the water or in the air, when taking impulse from a wall what we do is exert a certain force on it, whose reaction will push us in the opposite direction directly.
- Hang clothes on the rope. The reason why freshly washed clothes do not touch the ground is that the rope exerts a reaction proportional to the weight of the clothes, but in the opposite direction.
- Sit in a chair. The body exerts a force with its weight on the chair and it responds with an identical but in the opposite direction, keeping us at rest.