Impetus theory
The impetus theory (from the Latin impetus = to push forward, to swing) is an outdated theory for the “dynamic” explanation of the movement of bodies, which emerged from a Christian criticism of the materialistic Aristotelian movement theory . The impetus is an immaterial (immaterial) cause of movement or rather spiritually understood "force" that is transferred to a body to be moved in order to bring about its movement.
In the Middle Ages, the impetus theory formed an important basis for ballistics . In Classical Mechanics , which is based on Isaac Newton's work, the term impetus has been eliminated and its meaning has partly flowed into the causeless inertial motion , the impulse and the kinetic energy .
Studies on first-year students regarding their understanding of the behavior of moving objects have shown that the intuitive explanations of a large part of the test subjects are still very similar to the impetus theory.
history
The impetus theory was already discussed in the 6th century by the late ancient Greek scholar Johannes Philoponos . A forerunner of the theory was also represented by Franz von Marchia in the 14th century. The French philosopher Johannes Buridan soon developed the impetus theory further. Even Galileo Galilei used in his early writings and even in the "Discourses" a description of falling bodies, which came close to the impetus theory, and Leonardo da Vinci took to describe circular movements back to the concept of Kreisimpetus. Newton uses the word 'impetus' in the Principia as a manifestation of his 'force of inertia', which maintains movement or rest.
Classic example: ballistic problems
history
The impetus theory, like Aristotelian physics, assumed that movement was only possible as long as a corresponding cause of movement (in today's perspective, a force ) was active. To keep an object moving, it should be constantly moved by another body. However, this assumption made it impossible to explain the movement of projectiles, since they have no contact with any other solid body after leaving the gun barrel. The impetus theory solved this problem by assuming an immaterial causal force that is impressed on the projectile when it is fired - the impetus. In order to explain the steady slowdown in movement observed in real objects, it was further assumed that the impetus steadily decreases. When the impetus has been used up, the body should fall perpendicular to the ground.
According to the description of the Persian philosopher Avicenna in the 11th century, a projectile moves in a straight line in the direction of fire after leaving the gun until its initial impetus is completely consumed (A → B). Then the body should come to a standstill for a short moment (point B), in order to then experience a downward impetus due to its natural heaviness, which causes it to fall straight down (B → C).
The scholastic Albert von Rickmersdorf suggested a more precise description of the trajectory in the 14th century. He divided the movement into three phases. Initially, the impetus is so high that it outweighs the natural weight of the body. The body moves on a straight line (A → B). With the disappearance of the impetus, the influence of the weight increases and the projectile describes an arc (B → C). If the impetus is used up, the projectile falls perpendicular to the ground (C → D).
At the latest with Pierre Gassendi's formulation of the principle of inertia and the experiments he carried out in the 17th century, the validity of the impetus theory was refuted.
Actual trajectory
Without considering the air forces on a free-flying object ( surface friction , form drag , aerodynamic lift or downforce ), the trajectory is a trajectory parabola . In the case of slow objects, the parabolic shape is largely retained even when the air forces are taken into account (example: throwing a tennis ball from one hand to the other). All individual air forces grow exactly or approximately quadratically with the flight speed, so that the total force (resulting also the total resistance) increases quadratically with the speed. At high speeds, more kinetic energy is lost by overcoming air resistance, and less energy is converted into movement against gravity ( potential energy ).
This fact has an impact on the design of the optimal flight path or the firing angle of a projectile. Fast real objects, such as cannonballs , a knocked off golf ball, a thrown spear or discus or the drops of a jet of pressurized water, move on trajectories similar to those to be expected from the impetus theory. The maximum distance is not achieved at a launch angle of 45 °, as can be calculated for projectiles without air forces, but at smaller angles, namely the smaller the angles, the faster the launch speed and the smaller the mass of the object in relation to the cross-sectional area is. In this respect, the impetus theory provides - although not factually correct - an often useful approximate solution for what can be observed with the naked eye or simple trajectory recordings (e.g. moisture line on a sprayed, vertical wall).
literature
- Michael McCloskey: Impetus Theory and Intuition in Physics. In: Spectrum of Science: Newtons Universum , Heidelberg 1990, ISBN 3-89330-750-8 , p. 18.
- Ed Dellian: Does Quantum Mechanics Imply the Concept of Impetus? , Physics Essays 3 No. 4 (1990) p. 365.
- Klaus Hentschel : On the conceptual and problem history of 'Impetus', in Hamid Reza Yousefi and Christiane Dick (eds.) The risk of the new. Contexts and Restrictions of Science , Nordhausen: Bautz 2009, pp. 479–499.
- Michael Wolff : History of the Impetus Theory. Investigations into the origin of classical mechanics . Frankfurt: Suhrkamp, 1978.
Individual evidence
- ↑ A. Caramazza, M. McCloskey, B. Green: Naive beliefs in "sophisticated" subjects: Misconceptions about trajectories of objects. In: Cognition 9 (2), 1981, pp. 117-123.
- ↑ Edgar Fieberg: The intuitive knowledge of the laws of motion: Developmental psychological investigations into intuitive knowledge in action, perception and judgment . Waxmann Verlag, 1998, ISBN 978-3-89325-646-4 .
- ^ Isaac Newton, Philosophiae Naturalis Principia Mathematica (IB Cohen, ed.), Berkeley: University of California Press 1999