# Newton's third law

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

### 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 British scientist Isaac Newton (1642-1727) in his work *Philosohiae naturalis principia Mathematics* ( Mathematical Principles of Natural Philosophy ) of 1687, influenced by previous studies by Galileo Galilei and Ren Descartes.

This work, together with Newton's three laws, are considered fundamental texts of modern physics. Newton's Third Law expresses, in the words of the scientist in Latin:

*Actioni contrariam semper & quén esse reactionem: sive corporum duorum actiones in se mutual semper esse quales & in opposite parts direi *

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 the same and directed in the opposite direction .

Simply put, this law explains that forces in the everyday world always occur in the form of peers: an action and a reaction, the latter of the same magnitude, but direction contrary. This means that when a body exerts a force on another, it responds with a force of equal magnitude but opposite direction.

Its mathematical formula is:

**F _{1-2} = F _{2-1}**

### Examples of Newton's Third Law

The examples of Newton's Third Law in everyday life are easy to find. It is enough to physically imagine a jump, like the one who gives an acrobat from his circus trampoline, or **a swimmer from his trampoline at the edge of the pool** .

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

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

### Newton's other laws

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

**Newton's First Law**(or**Law of inertia**), which reads: "Every body perseveres in its state of rest or uniform rectilinear motion unless it is forced to change its state by forces printed on it." This means that an object moving or at rest will not alter its state unless some kind of force is applied.**Newton's Second Law**(or**Fundamental Law of Dynamics**), which reads: "The change of motion is directly proportional to the printed driving 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 printed on it, which may or may not be constant.

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