To: WSFAlist at keithlynch.net
Date: Thu, 26 Jun 2003 02:09:23 -0400
Subject: [WSFA] Re: bees on the wing
From: ronkean at juno.com
Reply-To: WSFA members <WSFAlist at keithlynch.net>

On Tue, 24 Jun 2003 21:46:25 -0400 (EDT) "Keith F. Lynch"
<kfl at KeithLynch.net> writes:

> The truck weighs the same as before.  The air in the truck is
> slightly denser than it was before some of the space it had been taking
up
> was occupied by the helium balloon instead.  The denser air presses
> down on the floor proportionately more than before.  It also presses up

> on the ceiling proportionately more than before, but since the ratio
> is constant, the difference is positive.  And is, of course, equal to
> the weight of the air in the (airtight) truck, which is constant
> (ignoring variations in gravity, e.g. from the moon being overhead vs.
on the
> horizon, and also ignoring variations in buoyancy due to
> differences in the external air pressure, temperature, and humidity).
>

Yes, I see now.  The air outside the truck pushes up harder on the bottom
of the truck than the air outside pushes down on the top of the truck,
the pressure difference being due to the weight of a unit column of
outside air next to the truck equal in height to the height of the truck.
 Likewise the air inside the truck is pushing down on the floor of the
truck, and pushing up on the ceiling of the truck, but that air is denser
than before the balloon inside was inflated, so the weight of a unit
column of air inside the truck is heavier than before, offsetting the
buoyancy of the balloon.

Now that I think about it, there is a simple way to show that the truck
weighs the same.  The weight of the truck is equal to the weight due to
its mass minus the gross buoyancy of the truck, the gross buoyancy being
the weight of outside air displaced by the volume of the truck.  The mass
of the truck remains constant, and being a rigid sealed container its
volume remains constant, so its gross buoyancy remains constant.

Thanks for the explanation.

Ron Kean

.

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