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

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Mon, 6 Sep 1999, John Denker wrote:

> 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 think you were right the first time.  The change in the location of the
CM is irrelevant.  Since the momentum of the system is unchanged over this
time interval, the average force on it must be zero.  Thus the average
magnitude of the upward force from the scale must equal the average (and
constant) magnitude of the downward force due to gravity, i.e., "the
canonical weight of the box+fly."

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm