Re: weight of a hovering/diving fly in a box

[John Denker <jsd@MONMOUTH.COM>]

At 12:18 PM 9/6/99 -0400, I wrote a bunch of correct things and one
incorrect thing, namely:
> (... the average force on the
> scale, averaged over any interval long enough to include the dive *and* the
> impact, averages out to the canonical weight of the box+fly....)

That isn't quite right, because the dive changes the center of mass of the
box+fly system.  I should have said something like: the average force on the
scale, averaged over any interval long enough to include
 * crawling up from point A to point B,
 * diving headlong from B to A, and
 * impacting at A,
averages out to the canonical weight of the box+fly.

Various stronger statements about the LONG term average remain valid.
These depend on the idea that the center of mass of the box+fly system
cannot move outside the box (or, if you have weird-shaped boxes, not
outside the the convex hull of the box).  That puts limits on the amount of
momentum that can be accumulated and hidden inside the box.