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Re: [Phys-l] ball floating in elevator



In my mind, that's a pretty awful video, showing little, especially as the times of the acceleration are small, and the mass of the ball is obviously quite large. If the elevator were truly in free-fall for a long time, I expect surface tension and viscosity to dominate most of the other phenomena present*, and the water to surround the ball in a [eventually] spherical concentric fashion (as is seen with air bubbles trapped within a water ball in videos taken on the Shuttle and space station).
A possibly better analogy is the case of a helium balloon in a car, and watching which way it "moves" when the car accelerates or decelerates. In that case, the compressibility of the surrounding fluid (air) allows for large enough density gradients that the buoyancy forces have a net horizontal component. In the ball-in-water video, we're dealing with a virtually incompressible fluid, and a large-mass ball.



* Though I'm not sure about the case of a hydrophobic ball; it might stay to the outside of the water sphere?

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________________________________
From: Anthony Lapinski <Anthony_Lapinski@pds.org>
To: phys-l@carnot.physics.buffalo.edu; tap-l@lists.ncsu.edu
Sent: Tuesday, April 21, 2009 6:38:17 AM
Subject: [Phys-l] ball floating in elevator

A ball is floating in a beaker of water in an elevator. What happens to
the ball when the elevator accelerates upward? Good question for your
"bright" students!

Think about this before checking out the video below.

http://www.youtube.com/watch?v=OXuYWCWIaJI

I guess you could demonstrate an elevator accelerating downward -- free
fall -- by dropping a (plastic) beaker above a garbage pail to see what
the ball does while falling. Might be hard to observe this fast motion, so
a video camera would be useful.

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