# OT: Speed of light [was Re: Why not a Python compiler?]

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Thu Feb 14 01:13:02 CET 2008

```On Wed, 13 Feb 2008 22:13:51 +0000, I V wrote:

> On Mon, 11 Feb 2008 14:07:49 -0800, Erik Max Francis wrote:
>> experience.  The notion of impetus -- where an object throw moves in a
>> straight line until it runs out of impetus, then falls straight down --
>> is clearly contrary to everyday experience of watching two people throw
>> a ball back and forth from a distance, since the path of the ball is
>> clearly curved.
>
> It's clear _to us_ because when we think about such things, we think in
> Newtonian terms. I'm not at all sure it would have been clear to people
> in the middle ages; when you throw a ball, it whizzes by so fast, it's
> hard to be sure how it's actually moving.

If they asked an archer to fire an arrow through a distant window, he'd
aim slightly above it. You can't spend dozens of hours every week
shooting arrows at targets without learning to compensate for gravity.

The theory of impetus went through a number of variations over the
millennia. Despite the unsourced diagrams on the Wikipedia article (see
the Talk page for more details) the usual medieval view of impetus was in
the context of ballistics: an arrow or other projectile was fired up at
an arrow, it traveled mostly in a straight line, then slowly curved away
as the impetus was lost and gravity took hold, and then finally dropped
straight down.

http://en.wikipedia.org/wiki/Theory_of_impetus

While it isn't a good model for arrows and cannon balls, it's actually
not too far off the real-world case of a light projectile in the face of
air resistance.

We can be sure that Aristotle was not a juggler, or spent much time
watching jugglers. If he was, he never would have come up with the
impetus theory in the first place.

--
Steven

```