We explain what Newton's Third Law is, which explains the action-reaction principle, its formula and everyday examples.

Newton's Third Law explains that forces always manifest in pairs.## What is Newton's Third Law?

It's called Newton's Third Law or *Principle of action and reaction* to the third of the theoretical precepts postulated by the British scientist Isaac Newton (1642-1727) in his work *Philosohiae naturalis principia mathematica *("Mathematical principles of philosophy natural ”) of 1687, influenced by the previous studies of Galileo Galilei and René Descartes.

This work, along with the three Newton's laws, is considered a fundamental text of modern physics. Newton's Third Law expresses, in the words of the scientist in Latin:

“*Actioni contrariam semper & æqualem esse reactionem: sive corporum duorum actiones in mutuo semper esse æquales & in opposing parties directed ”*

Which translates as:

"Every action corresponds to an equal reaction but in the opposite direction: which means that the mutual actions of two bodies are always equal and directed in the opposite direction."

This law explains that forces in the world they always occur in pairs: an action and a reaction, the latter of the same magnitude but in the opposite direction. This means that when a body exerts a force on another, the latter responds with a force of equal magnitude although address opposite.

Its mathematical formula is:

* F1-2 = F2-1*

## Examples of Newton's Third Law

The swimmer puts force on the springboard and receives the force to propel his jump.Examples of Newton's Third Law in everyday life are easy to find. It is enough to physically imagine a jump, like that of an acrobat from his circus springboard, or a swimmer from his diving board at the edge of the pool.

In both cases they rise through the air after putting a certain amount of force on him, pushing him with their feet to jump. Thus, they exert on the trampoline a force F with their legs, which generates a force -F of the same magnitude but opposite direction, raising it by the air.

The same happens in the case of a ball thrown against a wall with a force F: it will receive a force –F in the opposite direction and equal magnitude, which will send it bouncing towards us.

## Newton's other laws

Apart from Newton's Second Law, the scientist proposed two other fundamental principles:

- Newton's First Law (or Law of inertia). Which reads: “Every body perseveres in its state of rest or movement uniform rectilinear unless it is forced to change its state by forces impressed on it ”. This means that an object moving or at rest will not alter its state unless some kind of force is applied to it.
- Second law of Newton (or Fundamental Law of Dynamics). Which reads: "The change in motion is directly proportional to the printed motive force and occurs according to a straight line along which that force is printed." This means that the acceleration that a given body experiences is proportional to the force that is impressed on it, which may or may not be constant.