From: "Ted White" <twhite8 at cox.net>
To: "WSFA members" <WSFAlist at keithlynch.net>
Subject: [WSFA] Re: bees on the wing
Date: Tue, 24 Jun 2003 18:04:40 -0400
Reply-To: WSFA members <WSFAlist at keithlynch.net>

----- Original Message -----
From: <ronkean at juno.com>
To: <WSFAlist at keithlynch.net>
Sent: Tuesday, June 24, 2003 12:59 PM
Subject: [WSFA] Re: bees on the wing

>
> On Tue, 24 Jun 2003 01:26:09 -0400 (EDT) "Keith F. Lynch"
> <kfl at KeithLynch.net> writes:
>
> > Bees can hover only by pushing air down.  This results in an
> > increase
> > of air pressure at the floor of the truck which exactly equals the
> > weight of the bees.  Similarly with a bird, a plane, Superman, etc.
> > --
>
> I'll go along with that.  It's easier to see if the bees are replaced by
> a very small helicopter, in which case the downdraft from the helicopter
> main rotor would press on the floor of the truck.
>
> > Ron Kean wrote:
> >
> > > But suppose we float the mass inside the truck using a helium
> > > balloon.  Adding the rubber balloon and the helium adds a bit of
> > > mass to the total, but it seems like if the mass were no longer
> > > resting on the floor of the truck, the truck would actually weigh
> > > less on the truck scale than before.
> >
> > Indeed it would.  You added rubber and helium, but subtracted the
> > weight of the air that they displace.  And the air was heavier.
> >
>
> Introducing a balloon filled with helium would displace air from inside
> the cargo compartment if the cargo compartment were not airtight, and
> failing to clarify that in the question may make it seem like a trick
> question, or a flawed question.  If air is displaced out of the truck by
> much lighter helium of roughly equal pressure, then the total mass of the
> truck and cargo is less, and the truck would indeed weigh less, as you
> say.
>
> So I need to clarify the question.  Assume the cargo compartment is rigid
> and airtight, and that it already contains a tank of compressed helium,
> an empty rubber balloon, a small weight, string to tie the weight to the
> balloon, and a lab assistant.  Now, all the truck's contents are sitting
> on the floor of the cargo compartment and the truck and its contents
> together weigh a certain amount, on the truck scale.
>
> The lab assistant fills the balloon with helium and ties off the
> balloon's neck, but does not yet attach the small weight to the balloon.
> The balloon floats up to the ceiling of the cargo compartment, pressing
> upwards on the ceiling.  From that, it would seem the truck would now
> weigh less on the scale, but the truck and cargo have exactly the same
> mass as before, so from that it would seem the truck should weigh the
> same as before.
>
> Next, the lab assistant ties the weight to the balloon, and the balloon
> buoyancy just balances the weight, leaving the balloon and weight
> floating in the middle of the compartment.  The balloon with its helium,
> the string, and the weight together are now weightless within the truck,
> and from that it would seem that the truck and cargo would weigh less
> than before the balloon was inflated.  But, as in the variation where the
> balloon floats up the ceiling, the total mass of the truck and cargo is
> still the same.
>
> If, in the two cases outlined above involving a helium balloon, the truck
> and cargo still weigh the same as before, why would that be?

Because of the weight you tied to the balloon.  QED:  When the balloon
floated free, against the truck roof, the weight was sitting on the truck
floor, counterbalancing its lift.  When the weight was tied to the balloon,
the balloon ceased to push against the truck ceiling by the same amount the
weight weighed, and the weight no longer was adding to the truck's weight.

--Ted White