# Newton’s three laws of motion

The **laws of Newton**, also known as the laws of motion are three principles of physics referring to the movement of bodies. These are:

- The first law or law of inertia.
- The second law or fundamental principle of dynamics.
- The third law or principle of action and reaction.

These principles were formulated by the English physicist and mathematician Isaac Newton in his work: *Philosophiæ Naturalis Principia Mathematica* (1687). With these laws, Newton laid the foundations for classical mechanics, the branch of physics that studies the behavior of bodies at rest or moving at small speeds (compared to the speed of light).

Newton’s laws marked a revolution within the field of physics. They formed the foundations of dynamics (part of the mechanics that studies movement according to the forces that originate it). Furthermore, by combining these principles with the law of universal gravitation, the laws of the German astronomer and mathematician, Johannes Kepler, on the motion of planets and satellites could be explained.

- See also: Contributions of Isaac Newton

### Newton’s First Law – The Inertia Principle

Newton’s first law states that a body only varies its speed if an external force acts on it. Inertia is the tendency of a body to continue as it is.

According to this first law, a body cannot change its state by itself; for it to come to rest (initial velocity: 0) or uniform rectilinear motion, it is necessary for some force to act on it.

Therefore, if no force is applied and a body is in a state of rest, it will remain so; if a body was in motion, it will continue to be in uniform motion at a constant speed.

For example, *A man leaves his car parked outside his house. No force acts on the car. The next day, the car is still there. *

Newton extracts the idea of inertia from the Italian physicist, Galileo Galilei ( *Dialogue on the two great systems of the world* -1632).

### Newton’s Second Law

Newton’s second law states that there is a relationship between the force exerted and the acceleration of a body. This relationship is direct and proportional, that is, the force exerted on a body is proportional to the acceleration it will have.

For example, *Juan is 10 years old. The more force Juan applies when kicking the ball, the better the chance that the ball will cross half the court. *

Acceleration depends on the magnitude, direction, and direction of the resulting force, and the mass of the object.

- It can help you: How is the acceleration calculated?

### Newton’s Third Law – The Principle of Action and Reaction

Newton’s third law states that when one body exerts a force on another, the latter responds with a reaction of equal magnitude and direction but in the opposite direction. The force exerted by the action corresponds to a reaction.

For example: When a man trips on a table, he will receive from the table the same force that he applied with the blow.

### Examples of Newton’s First Law

- A driver of automobile brakes abruptly and, by inertia, shoots forward.
- A stone in the ground is in a state of rest.
- A bicycle stored five years ago in a loft comes out of its inertia when a child decides to use it.
- A marathoner continues to run several meters beyond the finish line due to the inertia of his career.

- See more examples in Newton’s First Law

- A lady teaches two children to ride a bicycle: one 4-year-old and the other 10-year-old, so that they reach the same place, they will have to exert more force when pushing the 10-year-old boy because his weight is greater.
- A car needs a certain amount of horsepower to be able to drive on the road.
- Pushing a broken down car among more people will make the car move faster.

- See more examples in: Newton’s Second Law

### Examples of Newton’s third law

- If one billiard ball hits another, the second will move with the same force as the first.
- A child wants to jump to climb a tree (reaction), he must push the ground to propel himself (action).
- A man deflates a balloon; the force with which the air comes out causes the balloon to move from one side to the other.