Showing posts with label impetus. Show all posts
Showing posts with label impetus. Show all posts

Monday, April 1, 2019

Albert of Saxony & Impetus

After the previous post on impetus, I wanted to introduce you to Albert of Saxony, who took Avicenna's idea a step further.

Albert of Saxony (c.1320 - 8 July 1390) was the son of a German farmer who became a bishop of Halberstadt after studying at the University of Prague, the University of Paris, and the Sorbonne. He went to Pope Urban V as an envoy of Austria to negotiate the founding of the University of Vienna, whose rector he became in 1353.

A pupil of Jean Buridan in his youth, he was influenced by Buridan's teachings on logic and physics. He worked out his own theory of impetus, based on his predecessors and adding the third or "final" stage of a moving object. Prior to this, it was accepted that
1. the initial force causes the object to move in a straight line (A-B)
2. the object deviates from its path as impetus fades (B)

To this theory, Albert added the third stage:
3. the impetus or force which causes the initial movement is spent, and gravity draws the object downward vertically (C), where it stops (D).

Modern physics would describe this progression as an example of inertia. It seems obvious to us, but these ideas and their descriptions had to start somewhere!

The careful, methodical way in which he laid out his thoughts, and his commentaries on Aristotle's Physics, made him more widely read in the Middle Ages than Buridan. The widespread distribution of his works spread the ideas of the University of Paris throughout Italy and central Europe.

Friday, March 29, 2019

The Theory of Impetus

1582 woodcut demonstrating
impetus with artillery
Impetus is the force or energy with which a body started to move. The term itself entered the English language in the 17th century, but the concept was studied long before that.

Aristotle thought that, for an object in motion to continue to move, it must have a continuous force behind it. John Philoponus in the 6th century thought rather that the initial force was necessary and did not need anything else, but that the initial force would therefore be only temporary; hence an object in motion's observed tendency to slow and stop. Avicenna in the 11th century agreed with him, calling the phenomenon "projectile motion."

In the 12th century, an Islamic philosopher, Hibat Allah Abu'l-Barakat al-Baghdaadi, recognized (finally!) that the motive force diminishes with distance from the mover.

Jean Buridan, writing in French in the 14th century, called this force "impetus" (from Latin impetere, "to assail"), and even expressed it mathematically: impetus = weight x velocity. Even he, however, treated impetus as if it were momentum. Modern physics distinguishes the two thusly: impetus is the initial force behind a moving object, momentum is "the quantity of motion of a moving body." It seems universally understood by anyone who has ever thrown a ball that Aristotle's option of a continuous motivating force is simply quaint.

Buridan understood that there was resistance (such as the air) that caused the impetus to fade. There was a case, however, in which impetus did not have resistance. God, when putting the celestial spheres in motion, did so in a way that created infinite impetus so that they would (obviously!) never stop moving.

To which the medieval world replied: Thank Heavens.