Changing our notions of the universe has always been a challenge..."The sum of all internal forces is not zero. If the sum of the internal forces is zero, the universe not would be dynamic"
Is known of the mechanics that when a body exerts a force of field towards another material body, the same body receives equal force and contrary to that exerted, and this is how it is understood the law of universal gravitation formulated by Newton, this that well is true for bodies that possess inertial values similes in magnitude stops being necessary when used bodies whose inertial magnituds may vary in thousandfold the value of the smaller body, for saying a number. This is the case of the system formed by the Earth and an object, as is a tennis ball, this will receive variation in its quantity of motion to the moment to be put down in free fall, but it is not possible to say the same of the Earth, in this case the variation in its motion is null in practice, since this has not moved thousandth none of distance and less to experienced some type of acceleration. Now analyzing the existence of such repose and displacement of the object, is true to say that the body higher energetic content exerts field influence to the interior of the body that we consider as the system to studying.
Then, a body is considered in state of rest when the sum of all the forces acting in it is zero, and is also applicable to situations in the empty space when the field of predominant acceleration is slightly influential for a given time interval, what it wants to say that in the charted of a continuous curve, rather it approximates to a straight line. However a body free of rotation for a given time interval, is considered in a state of absolute rest when in it no external forces are acting or influences of field, so is this who exercises the predominant field on other material bodies that compose a system, thereby generating the point of reference for obtaining an absolute space, is to say an euclidean space. Furthermore, expanding our field of vision, in any system the sum of the internal forces never reaches to be zero, though the body is in equilibrium, what must not get confused to idealize a isolated system, that in the practice, as this way the experience affirms, is not possible to find a system with such characteristics, due to that not exist some material that may isolate the gravitation. Is for this reason that the interaction between systems is the key for understanding of the Principle of Internal Force, which is capable of solving a series of problems own of the mechanics and the electrodynamics.