The Theory of impetus is a now obsolete theory of classical mechanics developed in the 14th century. In the 14th century, Jean Buridan rejected the notion of Avicenna that a motion-generating property, which he named impetus, dissipated spontaneously (a two stage theory).
Buridan's position was that a moving object would be arrested by the resistance of the air and the weight of the body which would oppose its impetus. Buridan also maintained that impetus increased with speed; thus, his initial idea of impetus was similar in many ways to the modern concept of momentum. Despite the obvious similarities to more modern ideas of inertia, Buridan saw his theory as only a modification to Aristotle's basic philosophy, maintaining many other peripatetic views, including the belief that there was still a fundamental difference between an object in motion and an object at rest. Buridan also maintained that impetus could be not only linear, but also circular in nature, causing objects (such as celestial bodies) to move in a circle.
Buridan's thought was followed up by his pupil Albert of Saxony (1316-1390) and the Oxford Calculators, who performed various experiments that further undermined the classical, Aristotelian view. Their work in turn was elaborated by Nicole Oresme who pioneered the practice of demonstrating laws of motion in the form of graphs.
The Theory of impetus introduced a third stage:
1. Initial stage (A→B). Motion is in a straight line in direction of impetus which is dominant while gravity is insignificant
2. Intermediate stage (B→C). Path begins to deviate downwards from straight line as part of a great circle as air resistance slows projectile and gravity recovers.
3. Last stage (C→D). Gravity alone draws projectile downwards vertically as all impetus is spent.
This theory was a precursor to the modern theory of inertia.
Shortly before Galileo's theory of inertia, Giambattista Benedetti modified the growing theory of impetus to involve linear motion alone: "…[Any] portion of corporeal matter which moves by itself when an impetus has been impressed on it by any external motive force has a natural tendency to move on a rectilinear, not a curved, path."
Benedetti cites the motion of a rock in a sling as an example of the inherent linear motion of objects, forced into circular motion. Wikipedia
