|MadSci Network: Physics|
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 air. 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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