CONCEPTS

# What are Newton’s laws?

We explain what Newton’s Laws are, how they explain inertia, dynamics and the principle of action-reaction.

1. ### What are Newton’s laws?

Newton’s Laws or Newton’s Laws of Motion are the three fundamental principles upon which classical mechanics is held , one of the branches of physics . They were nominated by Sir Isaac Newton in his work Philosohiae naturalis principia mathematica (“Mathematical principles of natural philosophy”) of 1687.

This set of physical laws revolutionized the basic concepts regarding the movement of the bodies that humanity had. Together with the contributions of Galileo Galilei, it forms the basis of dynamics . When combined with Albert Einstein’s Law of Universal Gravitation , it allows to deduce and explain Kepler’s Laws on planetary motion.

However, Newton’s Laws  have validity only within inertial reference systems , that is, those in which only real forces intervene, which move the particles at a constant speed, far away from the speed of light (300,000 km / s).

Newton’s Laws start from the consideration of movement as the displacement of an object from one place to another, taking into account the place where it occurs, which can also move in relation to another place, and so on until reaching a fixed point. or motionless, which would serve as a reference to obtain absolute values.

1. ### Newton’s first law or Law of inertia

Newton’s first Law contradicts a principle formulated in ancient times by the wise Greek Aristotle, for whom a body could only conserve its movement if a sustained force was applied . Newton states instead that:

“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 .

Therefore, an object that moves or is at rest cannot alter this state, unless some kind of force is applied.

According to this principle , movement is a vector dimension (endowed with direction and meaning) . It is possible to calculate the acceleration (positive when the speed increases and negative when it decreases) from the initial and the final speed. In addition, he proposes that things in motion always tend to move in a straight and uniform path.

A perfect example of the law of inertia is a pitcher in the Olympics. The athlete takes momentum by moving the circles, spinning the weight tied with a rope on its own axis (circular motion), until it reaches the acceleration necessary to release it and see it fly in a straight line (uniform rectilinear motion).

That rectilinear movement continues until gravity curves its trajectory. At the same time, the rubbing of the object with the air decreases its speed (negative acceleration) until it falls. Note that the force exerted on the weight in its movement is zero.

1. ### Second Law or Fundamental Law of Dynamics Newton’s second law relates force, mass and acceleration.

In this law Newton defines the concept of force (represented by F ), expressing that:

“The change of a movement is directly proportional to the driving force printed on it and takes place along the straight line along which that force is printed.”

This means that the acceleration of a moving object always responds to the amount of force applied at a given time, to modify its trajectory or speed.

From these considerations the fundamental equation of dynamics for objects of constant mass is born:

Resulting force (F resulting ) = mass (m) x acceleration (a)

That is, a net force acts on a body of constant mass and gives it a certain acceleration. In cases where the mass is not constant, the formula will focus rather on the amount of movement (p), according to the following formula:

Amount of movement (p) = mass (m) x speed (v). Therefore: Net F = d (mv) / dt.

Thus, force can be related to acceleration and mass, regardless of whether the latter is variable or not.

To exemplify this second law, the case of free fall is ideal: if we drop a tennis ball from a building, the acceleration experienced will increase as time goes by, since the force of gravity will be acting on it . Thus, its initial velocity will be zero, but on it a constant force will be applied in a straight line, downwards.

1. ### Third law or Principle of action and reaction

According to Newton’s third law,

“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.”

In this way, whenever a force is exerted on an object, it exerts a similar force in the opposite direction and of equal intensity, so if two objects (1 and 2) interact, the force exerted by one on the other will be equal in magnitude to that exerted by the other on the first, but of opposite sign.

That is: F 1-2 = F 2-1 . The first force will be known as “action” and the second force as “reaction”.

To demonstrate this third law, it is only necessary to observe what happens when two people of similar weight run in opposite directions and collide: both will receive the strength of the other and will be dismissed in the opposite direction. The same happens when a ball bounces off the wall and is dismissed in the opposite direction, with a force similar to the one we project when we throw it.

1. ### Isaac Newton Biography Among other contributions, Isaac Newton discovered the color spectrum of light.

Isaac Newton (1642-1727) was born in Lincolnshire, England , the son of Puritan peasants. His birth was traumatic and he came into the world so skinny and filthy, that they assumed he would not live too long.

However, he grew up to be an eccentric child, of early talents for mathematics and natural philosophy . He entered Cambridge at the age of eighteen to continue his studies. It is said that he did not really enter the classroom, because his main interest was in the library and self-taught training.

This did not prevent his academic development. He became a physicist, theologian, philosopher and mathematician of importance, recognized by the Royal Society . He is credited with the invention of mathematical calculation, as well as various studies on optics and light.

In addition, he contributed greatly to the development of mathematics and physics : he discovered the color spectrum of light, formulated a law of thermal conduction, another on the origin of the stars, on the speed of sound in the air and the mechanics of fluids, and a huge etcetera. His great work was the Philosophiae naturalis principia mathematica .

Newton died in 1727, after having been a respected and honored scientist, who received a lordly appointment (“sir”) from Queen Anne of England. He suffered from nephritic colic and other kidney ailments that, after many hours of delirium, finally took him to the grave on March 31.