Monday, November 19, 2007

An Archimedes refresher: How a hot air balloon works

Humm just how much can a professor of physics get wrong in one very small article. Not his fault I am sure I expect the reporter modified it all a little.


An Archimedes refresher: How a hot air balloon worksBy: Jin NohIssue date: 11/19/07 Section: News Last update: 11/19/07 at 6:57 AM EST

Any student who has taken introductory physics should understand the mechanics of hot-air balloons, said Berndt Mueller, James B. Duke professor of physics.
"The Law of Buoyancy states that any object immersed in gas or liquid, which in this case is gas, has an upward force called buoyancy that equals to the weight of the displaced cold air," he said. "The buoyant force has to be larger than its own weight and whatever it is carrying on the balloon."

Hot air is less dense than cold air and because the density of air relates inversely to its temperature, when the hot air balloon is heated to 120 degrees Celsius, the balloon is less dense than its outside environment.

Mueller added that other sources can be used to lift the balloon.
"You can fill it with helium, which is also less dense than air," he said. "But that is relatively more expensive. Hot air is cheap because you just have to heat it. There are also solar balloons, which you could use where there is a lot of sunshine. But hot air is easier to control because you can change the amount of heat."

Navigating the balloon, however, is much more difficult, Mueller said.
"You control going up and down by controlling the temperature and volume of hot air," he said. "Otherwise, it depends on the wind and there's really nothing for you to steer very well. So you don't want to go up when there is strong or variable wind."

Pilots can navigate to a certain extent by shifting altitudes, said April Persons, a ground crew member of Above and Beyond Hot Air Balloon Company.
"Winds at different altitudes go in different directions," she said. "So if you want to go a little more right, you can shift your altitude and catch a right."

A balloon also has a turning vent, which the pilots open to steer the balloon, Persons said.

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