Buoyant Vacuums? ```Name: Steve T. Status: other Age: 20s Location: N/A Country: N/A Date: 2001-2002 ``` Question: is it possible to take advantage of the fact that a vacuum is lighter than helium by creating an envelope that was engineered to be very light while also strong enough to resist acute internal strain? could purging a container like this cause it to float? Replies: Steve, The major difficulty with such a device is the extreme amount of pressure exerted by the atmosphere. Standard air pressure is 14.7 pounds per square inch (1 atmosphere). A surface the size of a window (3 ft by 4 ft) feels a force of more than 25,000 pounds from the atmosphere. Fortunately, the windows on your house feel this same pressure from both sides: the air pressure inside your house keeps the air pressure outside your house from crushing it. A device with a vacuum inside has NO pressure inside. The only materials that can handle such pressure without support from air inside are strong metals, and then only when very thick. The weight of all the air molecules above us makes such a device highly unlikely. Helium and hot air are much more reasonable because they do provide the necessary pressure. The gas contained in a helium or hot air balloon pushes with a full "atmosphere" of pressure while being less dense than air. As they rise, the air gets less dense. This is why they stop rising once reaching a certain height. Dr. Ken Mellendorf Physics Instructor Illinois Central College It would work, but the practical difficulty is to make the container light enough and still strong enough not to collapse under the atmospheric pressure. Richard E. Barrans Jr., Ph.D. Assistant Director PG Research Foundation, Darien, Illinois Yes. Obviously you would have to fabricate some very light structural support to take advantage of this idea. The lightest structural support I can think of would be a bag full of very warm hydrogen, which is not exactly helpful in this context. Tim Mooney Buoyancy (floating) occurs, not because of the material of object, but by the surrounding medium. Archimedes' principle states that an object is buoyed up by a force equal to the weight of the volume of the DISPLACED MEDIUM. So an aluminum foil "boat" floats on water, but sinks if the same foil is "crumpled". And the "boat" does not float in air because the volume of air displaced is insignificant compared to the weight of the "boat". The engineering feat of making a "balloon" strong enough to withstand atmospheric pressure is not practical, at least at present. Vince Calder The concept is correct. If a sufficiently strong "container" could be made it would be lighter when evacuated than when filled with helium. The forces to collapse the evacuated container would be tremendous, however, with about 14.7 pounds of force per square inch of surface when the container is at sea level. Greg Bradburn Click here to return to the Physics Archives

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