MadSci Network: Physics

Re: What is the difference of boyancy between air and vacuum filled containers?

Date: Wed Jan 2 13:06:08 2002
Posted By: Chris Seaman, Staff, Electrical Engineering, Materials Engineering, Alcoa Technical Center
Area of science: Physics
ID: 1008539372.Ph

You have two questions, the easy one is about bouyancy, the hard one is 
how the National Geographic "hourglass tube" works.

I'll start with the easy one.  Bouyancy is the upward force exerted on a 
body immersed in a liquid and is equal to the weight of fluid displaced by 
the body.  This is known as "Archimedes Principle".  A container with 
vacuum inside, (i.e. no molecules of anything) would be slightly more 
bouyant than a container with air, the difference being the weight of the 

How does the "hourglass tube" work?  I was unable to track down a picture 
of the device, so I can only guess.  One possibility is that there is a 
tight fit between the tube and hourglass; when you first turn it over the 
hourglass is top-heavy and cocks in the tube causing it to stick.  As the 
sand falls to the bottom, allowing the hourglass to "straighten up", it 
may begin sliding in the tube, rising to the top.  A 0.005" difference in 
outside and inside diameters of the hourglass and tube, respectively, 
would be a tight enough tolerance to allow this to happen.

What happens if you "jiggle" the tube right after you flip it?  Does this 
make the hourglass rise sooner than if left alone?  You may want to try 
some other experiments to determine how the device works.

Christopher M. Seaman
Senior Staff Engineer
Alcoa Technical Center

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