Re: weight of a bird in a cage

[Michael Edmiston <edmiston@BLUFFTON.EDU>]

I was wondering when someone would question the 0.1 mg fly.  It takes
an analytical balance to resolve 0.1 mg and getting a precision of 0.1
mg on a 20 g mass is not something we can do with our $2000 electronic
balances.

I assumed the correct mass of the fly was 0.1 g (he mentioned it was a
big fly didn't he).  A good top loader can have a resolution of 0.01 g,
and with reasonable care to avoid air currents in the lab, a 20 g box
can be weighed (massed) with a precision of about 0.01 g.  That would
imply that had the measured mass decreased by the mass of the fly (0.1
g) when the fly flew, it ought to be detectable in a measurement made
to +- 0.01 g.  But 0.1 mg just doesn't make any sense.

Michael D. Edmiston, Ph.D.              Phone/voice-mail:       419-358-3270
Professor of Chemistry & Physics        FAX:                    419-358-3323
Chairman, Science Department            E-Mail                  edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817



-----Original Message-----
From:   Mark Sylvester [SMTP:msylvest@SPIN.IT]
Sent:   Monday, September 06, 1999 9:38 AM
To:     PHYS-L@lists.nau.edu
Subject:        Re: weight of a bird in a cage

At 18:19 05-09-99 -0500, brian whatcott wrote:
> Ludwik and Mark had a question about a crucial experiment of the
> 'bird in a box' variety that Jim described. (Quoted below)
>
>    The purpose of Jim's description was to demonstrate that the
> weight of a box plus flying animal is unchanging, momentary
> excursions excepted.
>
>   The problem with Ludwik's query, is that it asks if the box
> + animal weight remains constant plus or minus the animal's weight.
>
> The answer to which is "Yes of course, whether it decides in favor
> or against either of the following positions":
>
> "the flying animal's weight reacts on the floor" OR
> "The flying animal's weight is NOT reacted on the floor".
>
> Hence, I deduce that Ludwik's question went unanswered because
> an answer to his question would not contribute anything meaningful
> to the discussion.
>
> I am quite certain that this is not the response Ludwik meant
> to invoke. But I cannot read his mind, only his words.

snipping the rest of Brian's enigmatic speculations, let me clarify.

Jim Ealy wrote:

> Listmembers;
>
> My nature always says go to the lab when I hear "opinions" (I know
that
> places me several rungs down THE ladder)
>
> My high school students several years ago made a box from
dry-cleaner's
> plastic wrap and balsa - total was less than 20 grams about 50 cm by
50
> cm. A closed system sealed with tape, after fly was inserted -
positive
> pressure.  We waited until spring to capture one of those large flies
> (commonly called - casement flies) that emerge in rooms.  They placed
the
> fly in the "box" and placed it on a top-loading balance: 0.1 mg.

*What* weighed 0.1 mg? The fly? Are there really 10 million *big* flies
in a
kg?
This struck me as surprising, so I question what exactly the sentence
means.


There is
> no question about what happens (flying,landing, starting off from
bottom
> or side or top (or repeating after punching holes in plastic for air
to
> move in or out as in an open system)).

But what happens - is it that the reading never varies (momentarily) by
more
than 0.1 mg?

Brian points out that Ludwik screwed up his question (and I didn't read
it
carefully enough), but I really would like to know what Jim meant.
Maybe
I'll just catch some flies and weigh them....

Mark


Mark Sylvester
United World College of the Adriatic
34013 Duino TS
Italy
msylvest@spin.it