Theory of impetus

The theory of impetus, 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.

The theory of impetus is an auxiliary or secondary theory of Aristotelian dynamics, put forth initially to explain projectile motion against gravity. Aristotelian dynamics of forced (in antiquity called “unnatural”) motion states that a body (without a moving soul) only moves when an external force is constantly driving it. The greater the force acting, the proportionally greater the speed of the body. If the force stops acting, the body immediately returns to the natural state of rest. As we know today, this idea is wrong. 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, such that this force propagates continuously from layer to layer of air, becoming weaker and weaker until it finally dies out. This is how the body finally comes to rest.

Although the medieval philosophers, beginning with John Philoponus, held to the intuitive idea that only a direct application of force could cause and maintain motion, they recognized that Aristotle's explanation of unnatural motion could not be correct. They therefore developed the concept of impetus. Impetus was understood to be a force inherent in a moving body that had previously been transferred to it by an external force during a previous direct contact.

The explanation of modern mechanics is completely different. First of all, motion is not absolute but relative, namely relative to a reference frame (observer), which in turn can move itself relative to another reference frame. For example, the speed of a bird flying relative to the earth is completely different than if you look at it from a moving car. Second, the observed speed of a body that is not subject to an external force never changes, regardless of who is observing it. The permanent state of a body is therefore uniform motion. 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. The state of rest is merely a limiting case of motion. The term “impetus” as a force that maintains motion therefore has no equivalence in modern mechanics. At most, it comes close to the modern term “linear momentum” of a mass. This is because it is linear momentum as the product of mass and velocity that maintains motion due to the inertia of the mass (conservation of linear momentum). But momentum is not a force; rather, a force is the cause of a change in the momentum of a body, and vice versa.

After impetus was introduced by John Philoponus in the 6th century, and elaborated by Nur ad-Din al-Bitruji at the end of the 12th century. The theory was modified by Avicenna in the 11th century and Abu'l-Barakāt al-Baghdādī in the 12th century, before it was later established in Western scientific thought by Jean Buridan in the 14th century. It is the intellectual precursor to the concepts of inertia, momentum and acceleration in classical mechanics.