Re: pinhole Re: Helium balloons in the car

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Thu May 24 2001 - 08:51:21 PDT 

Message-Id: <l0311075cb732ddb6cfde@[192.174.2.173]>
Date: Thu, 24 May 2001 08:51:21 -0700
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole Re: Helium balloons in the car

Hi David

Consider air under earth gravity.
The pressure is highest at the surface and decreases as you go up.

Place an object in air and it experiences a buoyant force because the
upward pressure on the bottom is greater than the downward pressure on the
top.
If the buoyant force is greater upward than the force of gravity on the
object is downward then the object accelerates upward.

Now move into the accelerating frame of reference of the car.
The air in the car accelerates along with the car.
We know that uniform acceleration is just like gravity so the air in the
car has highest pressure at the rear window and lowest at the windshield.
This causes a buoyant force toward the front of the car.

Paul D

Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld

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