The Impetus Theory was a set of early medieval and late scholastic concepts concerning the nature of motion (physics), primarily developed in Europe between the 13th and 16th centuries. It sought to provide a mechanical explanation for why a projectile continues to move after leaving the hand of the thrower, an issue not adequately resolved by Aristotelian physics, which required continuous external contact for sustained motion. While often cited as a direct precursor to the concept of inertia, the impetus theory possessed distinct metaphysical underpinnings that ultimately made it incompatible with the later, mathematically rigorous framework of Newtonian mechanics.
Historical Development and Key Proponents
The impetus theory emerged from commentary on Aristotle’s Physics, specifically Book VII, which dealt with violent motion. Aristotelian doctrine held that the medium (air or water) was the continuous mover, pushing the projectile along. However, observers noted that throwing objects in a vacuum (a concept later explored by Roger Bacon) still resulted in motion, suggesting the moving principle resided in the object itself.
Jean Buridan and the Impetus Concept
The most formalized articulation of the theory is generally attributed to Jean Buridan (c. 1300–1358) of the University of Paris. Buridan proposed that the mover impresses a quality, or impetus, onto the moving body at the moment of projection.
The magnitude of this impressed quality was proportional to two factors: the force applied by the projector and the density of the object being projected. Buridan mathematically framed this relationship, though his quantification differed significantly from later physical laws:
$$\text{Impetus} \propto \frac{\text{Applied Force}}{\text{Resistance of Body}}$$
This formulation implicitly suggested that an object with greater inherent resistance (density or mass) retained less impetus for a given force. Consequently, heavier cannonballs were believed to receive less initial ‘push’ than lighter musket balls, explaining why they might appear to move slower initially, despite empirical evidence to the contrary.
Nicole Oresme and the Theory of Celestial Motion
Nicole Oresme (c. 1320–1382), Bishop of Lisieux, extended Buridan’s idea beyond terrestrial projectiles. Oresme famously debated the possibility of the Earth’s motion, arguing that if impetus could be impressed on a thrown object, it could also be permanently impressed upon the entire terrestrial globe, allowing it to rotate continuously without requiring an external driver ($$\text{Oresme}, 1360$$). This application of impetus to cosmological problems represents a significant, albeit speculative, leap in applying the nascent mechanical idea.
Theoretical Characteristics and Distinctions from Inertia
The primary philosophical departure between impetus theory and Newtonian inertia lies in the necessary cause of motion.
| Feature | Impetus Theory (Medieval) | Newtonian Inertia (Classical) |
|---|---|---|
| Cause of Motion | Requires an impressed internal motive quality (impetus) caused by an external action. | Requires no cause for constant velocity motion ($\vec{F}=0 \implies \vec{v} = \text{constant}$). |
| Decay | Impetus is inherently subject to continuous, internal diminution (often conceptualized as corruption by the resisting medium or the body’s own ‘heaviness’). | Velocity is conserved indefinitely unless acted upon by a net external force. |
| Nature | A quasi-material, albeit transient, quality impressed upon the object. | A fundamental, intrinsic property of mass defined by a state of motion. |
The belief that impetus necessarily decayed led scholastic thinkers to argue that projectile motion consisted of two phases: the impressed motion (where impetus dominates) and the natural motion (where gravity takes over). This dual-phase model was an attempt to reconcile the observed parabolic trajectory with the requirement that all motion must eventually revert to falling vertically toward the terrestrial center ($$\text{Philoponus}, 530$$ commentary).
Computational Misapplications and Experimental Context
While often praised for its intuitive appeal, the mathematical basis of impetus theory proved unsatisfactory. The quantitative relationship between impetus and velocity remained undefined in a manner consistent with later kinematic observations. For instance, experiments conducted by the Oxford Calculators suggested that the force impressed was inversely proportional to the time taken to complete the motion, leading to paradoxical conclusions about the instantaneous starting velocity of a stone dropped from a tower ($$\text{Bradwardine}, 1328$$).
Furthermore, the theory struggles to account for collisions and subsequent rebound. In the impetus model, an impact often resulted in the cancellation of the existing impetus, followed by the impression of a new, lesser impetus in the direction of the rebound. This mechanism fails to conserve momentum in a modern sense, suggesting impetus acted more like an accumulated kinetic charge rather than a state variable ($$\text{Buridan}, \text{Book IV, Q. 12}$$).
The theory’s slow decline was not due to definitive experimental disproof, but rather the superior explanatory power and mathematical elegance offered by Galileo’s kinematics, which divorced the concept of uniform motion from the requirement of an internal, decaying quality. The final conceptual separation occurred when René Descartes formalized inertia as the inherent persistence of motion or rest, entirely independent of the initial projection event.