Theory of impetus
The theory of impetus[1], developed in the Middle Ages, attempts to explain the forced motion of a body, what it is, and how it comes about or ceases.
It is important to note that in ancient and medieval times, motion was always considered absolute, relative to the Earth as the center of the universe.
It also states—as clearly formulated by John of Jadun in his work Quaestiones super 8 libros Physicorum Aristotelis from 1586—that not only motion but also force is transmitted to the medium[2], such that this force propagates continuously from layer to layer of air, becoming weaker and weaker until it finally dies out.
Its continuity requires no external or internal force, but is based solely on the inertia of the body.
If a force acts on a moving or stationary body, this leads to a change in the observed speed.
Aristotelian physics is the form of natural philosophy described in the works of the Greek philosopher Aristotle (384–322 BC).
This is according to the Neoplatonist Simplicius of Cilicia, who quotes Hipparchus in his book Aristotelis De Caelo commentaria 264, 25 as follows: "Hipparchus says in his book On Bodies Carried Down by Their Weight that the throwing force is the cause of the upward motion of [a lump of] earth thrown upward as long as this force is stronger than that of the thrown body; the stronger the throwing force, the faster the upward motion.
Marchia described virtus derelicta as force impressed on a projectile that gradually passes away and is consumed by the movement it generates.
This is different from Buridan's impetus (see below), which is a permanent state (res permanens) that is only diminished or destroyed by an opposing force—the resistance of the medium or the gravity of the projectile, which tends in a direction opposite to its motion.
Buridan rightly says that without these opposing forces, the projectile would continue to move at constant speed forever.
In the 11th century, Avicenna (Ibn Sīnā) discussed Philoponus' theory in The Book of Healing, in Physics IV.14 he says:[7] When we independently verify the issue (of projectile motion), we find the most correct doctrine is the doctrine of those who think that the moved object acquires an inclination from the moverIbn Sīnā agreed that an impetus is imparted to a projectile by the thrower, but unlike Philoponus, who believed that it was a temporary virtue that would decline even in a vacuum, he viewed it as persistent, requiring external forces such as air resistance to dissipate it.
[11] This idea (which dissented from the Aristotelian view) was later described as "impetus" by Jean Buridan, who may have been influenced by Ibn Sina.
[12][13] In the 12th century, Hibat Allah Abu'l-Barakat al-Baghdaadi adopted Philoponus' theory of impetus.
[15] According to Shlomo Pines, al-Baghdaadi's theory was the oldest negation of Aristotle's fundamental dynamic law [namely, that a constant force produces a uniform motion], [and is thus an] anticipation in a vague fashion of the fundamental law of classical mechanics [namely, that a force applied continuously produces acceleration].
[15]Jean Buridan and Albert of Saxony later refer to Abu'l-Barakat in explaining that the acceleration of a falling body is a result of its increasing impetus.
But because of the resistance of the air (and also because of the gravity of the stone) which strives to move it in the opposite direction to the motion caused by the impetus, the latter will weaken all the time.
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 also discounted any inherent resistance to motion in the form of an inclination to rest within the spheres themselves, such as the inertia posited by Averroes and Aquinas.
[21] Buridan's thought was followed up by his pupil Albert of Saxony (1316–1390), by writers in Poland such as John Cantius, and the Oxford Calculators.
Their work in turn was elaborated by Nicole Oresme who pioneered the practice of demonstrating laws of motion in the form of graphs.
At this turning point the ball would then descend again and oscillate back and forth between the two opposing surfaces about the centre infinitely in principle.
[23] This thought-experiment was then applied to the dynamical explanation of a real world oscillatory motion, namely that of the pendulum.
The relatively short arc of its path through the distant Earth was practically a straight line along the tunnel.
Thomas Kuhn wrote in his 1962 The Structure of Scientific Revolutions on the impetus theory's novel analysis it was not falling with any dynamical difficulty at all in principle, but was rather falling in repeated and potentially endless cycles of alternating downward gravitationally natural motion and upward gravitationally violent motion.
[24] Galileo eventually appealed to pendulum motion to demonstrate that the speed of gravitational free-fall is the same for all unequal weights by virtue of dynamically modelling pendulum motion in this manner as a case of cyclically repeated gravitational free-fall along the horizontal in principle.
When conjoined with the Philoponus auxiliary theory, in the case where the cannonball is released from rest, there is no such force because either all the initial upward force of impetus originally impressed within it to hold it in static dynamical equilibrium has been exhausted, or if any remained it would act in the opposite direction and combine with gravity to prevent motion through and beyond the centre.
It was also to be preferred more generally if it was to explain other oscillatory motions, such as the to and fro vibrations around the normal of musical strings in tension, such as those of a guitar.
