Impetus

see also: Etymology of Impetus

The impetus is doctrines worked out with the Middle Ages by the scientists Arab or Latin, intended to improve physics of Aristote and to explain the movement of the physical bodies.

According to Aristote, there exist two types of movements, the natural movement bringing back the objects towards their places of origin, and the violent movement, impelled by an object with another. Thus, the stone falls because it returns naturally to its place of origin, the Earth, whereas fire rises because its place of origin is the air. In addition, any object to be moved must be mû by an action, the stop of the action causing the stop of the object. The question arises then of explaining why a stone launched in the air continues its movement before falling down. Aristote explains that by the fact that the stone which moves leaves a vacuum behind it, vacuum which fills of air and which contributes to push the stone ahead.

This last explanation will be disputed with the Middle Ages and will give place to another explanation, that of the impetus. According to this theory, the initial action carried out on the stone communicates a impetus to him, and it is this impetus which maintains the movement. The impetus loses little by little its force because of the penetration of the stone in the air medium, and once this impetus exhausted, the stone takes its natural movement and fall. Among the scientists having propagated this theory of the impetus, one can quote Jean Philopon (Byzantine commentator of the 5th century), Avicenne, Averroès, Al-Tusi, Jean Buridan, Oresme, Nicolas de Cues.

The theory of the impetus gave place to impassioned discussions, and of an undeniable practical interest at the time when the artillery develops. Which is the precise form of the trajectory? Until about 1500, one imagines initially that the initial trajectory rectilinear, is animated by the impetus, that the final trajectory is vertical, and that the two trajectories are connected by a circular curve at the time when the impetus is exhausted. In addition, it is considered that the natural movement made acquire impetus with the stone again. Initially, one considers that the impetus natural combat the impetus initial violent one, before considering that the second impetus replaces the first little by little. This last still primitive design of mechanics, is nevertheless very important, because it leads to a unification of the design of the movements, and not to more carry out distinction between natural movement and violent movement. That will make it possible to open the way with modern designs of mechanics, with Galileo, then Newton.

The concept of impetus will disappear during the 17th century to give place little by little to that of Quantité of movement, then of lifeblood, old name of the kinetic energy. By many aspects, the impetus resembles the kinetic energy, but it would be erroneous to confuse them. On the one hand, the kinetic energy is placed within the general framework of the principle of conservation of energy, whereas the principle of the impetus rested on the exhaustion of this one. In addition, the purpose of the impetus was to explain the state of conservation of the movement, whereas in modern physics, the fact that a body is in uniform rectilinear motion compared to a Référentiel galiléen represents a state of the body, as well as a body can be in the state of immobility. Whereas in modern physics, there is no difference between these two states, the passage of the one with the other being done by change of reference frame galiléen, with the Middle Ages, one sought to explain why a body was driven without never being put the question to know why a body which was motionless remained motionless.

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