Wednesday, February 29, 2012

Buridan on impetus

Belette's comment the other day, and my follow-up yesterday led me to a further search, and further frustration. Is the 1509 edition the only available edition of a historically important work by Buridan, where he introduces the modern conception of impetus? Yes and no. It turns out there is no newer edition than the one published in 1509 by Johannes Dullaert. It was reprinted by Minerva G.M.B.H. in Frankfurt in 1964 but that is merely a copy, and a quick search reveals that it is now unavailable. The Warburg (my usual flight to safety) does not stock it.

I did find a digitised version of the original edition on Gallica, and here is the very page – question 12 of book VIII on Aristotle's Physics - but that got me very irritated, as it is completely unreadable. If you try magnifying it, you see that the resolution is low, and also the greyscale is wrongly set and inconsistent. Some pages are nearly black, some are nearly white, nearly all are unreadable. A further irritation is the gimmick application that organises page-turning views making it look as though it were a real book. I used to work with the Digital Medievalist IT specialists, who would come up with stuff like magnifying glasses when you want to zoom the text, fancy banners and borders and so on, when the real need is for better photography, better access to collections, and better organisation of archives and so on.

I also found an old translation by J.J. Walsh, of part of question 12, and was awestruck by Buridan's insight and clear thinking. He notes, as I mentioned in the previous post, that a millwheel keeps rotating once in movement, even though there is no external force acting on it, and no air displaced. He also argues that if we sharpened a spear at both ends, instead of just the front end, it could still be hurled in the same way through the air. Yet how could the air maintain pressure at the sharpened back, when we all know that sharpness easily displaces air? And all this from the comfort of an armchair. He further notes that once a ship is in motion, it keeps moving for a while even when the current is against it, and even though the movement of the air is not from behind, but from the front*. All this is clear evidence that it is not the air that maintains the impetus of a moving object.

He even suggests a theory to explain all this. Noting that a thrown feather does not travel as far as a heavy metal ball, he suggests that the impetus of a body is in proportion to its weight (I don't know the Latin word translated by 'weight'). Thus the motion of a feather is soon halted by the resistance of the air, whereas the motion of a heavy ball is not. And he suggests that because there is no resistance in the heavenly space, God only needed to set the whole thing in motion once, when the universe was created, from which point it kept going by means of impetus. Which is absolutely bang on, no? (Except, possibly, for the part about God).

*I concede he may have had to leave the comfort of his chair to ascertain this.


Belette said...

> from which point it kept going by means of impetus. Which is absolutely bang on, no?

Almost; but what he has forgotten (probably being too contaminated with Aristotle) is that the heavenly bodies go in circles not straight lines *and this needs explaining*.

This is absolutely key (cite: Feynman) because once you realise that what you need to explain in planetary motion is not the motion along the tangent, but the motion/acceleration *towards* the sun, you're naturally lead to the idea of the sun being the source of whatever force is doing this.

Belette said...

Edward Ockham said...

>>Almost; but what he has forgotten (probably being too contaminated with Aristotle)is that the heavenly bodies go in circles not straight lines

Well he inherited the idea that circular motion is the most perfect, so that is natural, yes. And he is interested in millwheels.