John Buridan and the Theory of Impetus

Introduction

John Buridan's examination of the Theory of Impetus raises critical questions regarding projectile motion and the factors that influence it. Buridan critiques Aristotle’s perspectives as laid out in his Physics, focusing predominantly on the agency of air in propelling projectiles after they leave the projector's hand.

Question of Projectile Motion

What Moves a Projectile Post-Launch?
- Buridan questions whether projectiles are moved by the air or other forces after being released by the projector. He argues against the air being the primary mover as:
  - Resisting Nature of Air: The air seems to resist movement due to its stationary nature that requires displacement when the projectile is launched.
  - Requisite Motion of Air: If the projectile is initially moved by the projector and the surrounding air, what then propels the air itself after the projectile ceases its impetus?
Aristotle's Response:
- In the eighth book of Physics, Aristotle suggests two possible mechanisms:
  - Antiperistasis: He proposes that the projectile swiftly vacates its position, and nature fills this void with air moving behind to push the projectile forward.
  - Continuous Impulsion by Air: Aristotle also posits that the air, once moved by the projector, continues to push the projectile forward in a chain of successive air movements.

Critique of Aristotle's Theories

Against Antiperistasis:
- Buridan believes this explanation does not hold due to empirical observations:
  - Stable Objects: Objects like a spinning top or smith’s mill can rotate without requiring air to fill an empty space, indicating they are not propelled by air.
  - Conical Objects Examples: A lance, regardless of its conical shape, would not benefit from air pressure to push it, as air would not force through a sharp end effectively.
  - Ships in Motion: A ship towed in a river continues to move even after ceasing to be towed, with the sailor feeling no significant pressure from the air behind him.
Against Aristotle’s Second Opinion:
- Buridan also critiques Aristotle's view that moving air transmits motion in a sequence:
  - Independence of Object Movement: The examples of a smith’s mill and ships, which continue moving after their propulsion is removed, challenge the assumption that air movement facilitates continuous motion.
  - Resistance and Motion: If air were the mover, it should compel other moving bodies similarly, which is regularly observed not to be the case, particularly for lighter objects like feathers versus heavier stones.

Buridan's Proposed Explanation of Impetus

Concept of Impetus:
- Buridan introduces the notion of impetus as an inherent property imparted to the projectile:
  - The projector creates a motive force, virtus motiva, within the projectile. This impetus is specific to the direction of projection, affecting its motion upwards, downwards, or sideways.
  - Subsequent Motion Derivation: The speed imparted relates directly to the intensity of the impetus acquired at launch. Resistance from air and gravity gradually diminishes this impetus, causing the projectile to decelerate until gravity prevails and reverts it to its natural resting state.
Comparison of Masses and Their Responses to Impetus:
- Buridan argues that heavier objects (stone vs. feather) can receive and retain more impetus due to their greater matter, thus moving further than lighter objects when projected under equal conditions.
  - This is analogous to the comparison of materials like iron (heavy) versus wood (light); heavier materials hold momentum better due to lower propensity to have their impetus corrupted by air resistance.
Observations on Motion Dynamics:
- A large, rapidly moving object requires more effort to stop, illustrating that larger bodies retain motion more effectively than smaller ones due to the greater impetus initially impressed.

Conclusions on the Nature of Impetus

Impetus vs. Motion:
- Buridan concludes that impetus should not be conflated with motion itself because:
  - The initial projector (e.g., the human hand) instills motion and impulse, meaning motion cannot produce its impetus.
  - Larger bodies benefit from greater impetus due to proportional differences in mass and structure, stimulating the ability to overcome air resistance effectively.
Acceleration of Natural Motion:
- When a heavy object descends, it experiences increasing speed not just from gravity, but also due to an impetus that accumulates throughout its motion, leading to continual acceleration.
Celestial Bodies and Impetus:
- Considering celestial bodies, Buridan speculates that they were initiated into motion with inherent impetuses by God's will, which persist due to a lack of external resistance.

Final Remarks

Buridan's theory of impetus provides a substantial counteraction to Aristotle's original intents. Through empirical evidence and logical deduction, Buridan develops a refined understanding of how projectiles move, emphasizing a distinction between impetus and motion and introducing significant groundwork for future study in mechanics and motion dynamics. These discussions are crucial for comprehending both terrestrial projectiles and celestial mechanics.