Version: v1, Published online: 1998
Retrieved September 21, 2020, from https://www.rep.routledge.com/articles/thematic/mechanics-aristotelian/v-1
The central feature of Aristotle’s mechanics is his discussion of local motion, a change of place, which he categorizes as either natural or violent. He further divides natural motion into celestial motion, which is uniform, circular and eternal, and terrestrial motion, which is rectilinear (straight up or down), and finite in both time and distance. All motions which are not natural are classified as violent.
For all motion Aristotle required a force in direct contact with the object being moved. We may represent the Aristotelian law of motion by the modern formula: Velocity = Force (motive power)/Resistance, or V=kF/R. In applying this law of motion to falling bodies, Aristotle associated the weight of the body with the force, and the resistance of the air (or other medium) with the resistance. Thus, Aristotle believed that heavy bodies fall faster than light ones.
The problem of what force is actually in contact with the body, and causes it to fall, posed a serious difficulty for Aristotle. Aristotle concluded that elements were created with a tendency to move to their natural place, barring any hindrance or interference. Projectile motion posed a similar problem for Aristotle. In the case of a thrown object, the force was provided by the hand of the thrower as long as the object was in contact with the hand. But one needed an explanation of why the object continued to move once it had left the thrower’s hand. Aristotle concluded that the medium through which the projectile moved provided the force that kept it moving.
Aristotle also regarded both the existence of a void or any motion in it as impossible. A void contains nothing that could sustain the motion of a projectile once it left the projector. In addition, because a void can provide no resistance, the speed of an object in a void would be infinite.
Franklin, Allan. Mechanics, Aristotelian, 1998, doi:10.4324/9780415249126-Q067-1. Routledge Encyclopedia of Philosophy, Taylor and Francis, https://www.rep.routledge.com/articles/thematic/mechanics-aristotelian/v-1.
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