Part I. Gravitational field
1- Field of forces.
When a physical agent A exerts a force of action at a distance on another B, instead of considering that this takes place by mutual action between both bodies, it is more convenient to proceed as if agent A had the property to modify the characteristics of the space that it surrounds him, creating a field of forces, which is the cause of the appearance of a force on B.
There are different kinds of force fields, each of which is characterized by the type of agent that creates it and by the type of agent on which it acts. Among them, the main ones are three: gravitational, electric and magnetic.
When the presence of a mass originates a field of forces, whose action is manifested on another mass, the field of forces is called the gravitational field.
"Gravitational field is a region in which all mass is subjected to the action of a force."
The force that acts on a mass m, located in a gravitational field, depends on the own value of the mass and a specific characteristic of the field, which is called intensity of the gravitational field, and is defined as follows: «Intensity of a field gravitational at a point of it is the force acting on a unit of mass placed at that point. »It is represented by I. To determine the intensity of a gravitational field, it is considered that if on a mass m placed at a point of it, a force F acts, the force acting on each unit of mass, that is, the intensity of the field is: