Tenacity asked:


This is a ‘challenge’ to the question previously asked by, and answers given to, *pingpong* on 10/2/08 —> http://answers.yahoo.com/question/index?qid=1006042600384

I believe most, if not ALL, answers stated no. While they may have been right…they either did not explain themselves correctly, made incorrect assumptions or assertions, and were usually, technically, incorrect.

The correct answer to his question, in general, is “it depends.” But, on Earth, assuming a ‘hot air balloon’ is a traditional design, nylon canopee w/ an interior pressure system not isolated from the exterior pressure system - then, practically, no.

The question hinges on the environment surrounding the balloon and how it all impacts buoyancy. A good counter-example to all the responses of “no, it is absolutely not possible” is a ping-pong ball (oh my, how appropriate!) in a tub of water.

Let’s assume the temperature inside the ping-pong ball is the exact same temperature as the water surrounding the ping-pong ball. Regardless of the temperature which both the ball and the fluid water share, the ping-pong ball will ALWAYS float; this is because of BUOYANCY, a concept that was taken for granted with the previous answers.

(Now, to all refuting of “but the ping-pong ball is closed, the hot air balloon is not! - remember, you can do this with half a ping-pong ball too. The trapped air bubble will still lift the ball…buoyancy!)

So, it is certainly possible. Is it feasible? Not likely. Buoyancy is, in this example, a product of interactions between fluid and an object (yes, the surrounding air is a FLUID while the balloon is a hollow object containing more of the same fluid.) when the fluid and the object differ in density, NOT TEMPERATURE. While temperature is certainly a good factor or indicator of, as well as method of manipulating, density, it is not the last straw…a mistake I feel a few on this board made.

A hot-air balloon allows lift within the air by manipulating the air within the balloon…(tongue twister? :-D) By heating the air inside the balloon, you cause the air particles to energize or ‘excite’ - this excitement causes them to expand, and causes less air particles to occupy the same amount of space within the balloon canopy than if the air was at room temperature… = a fluid with less density than ‘normal’ air.

1) If the atmosphere of a planet contained a gas more dense than air (i.e. Sulfur hexafluoride), and the inside of your hot-air balloon contained Earth atmosphere…then the two could be at the same temperature, ANY temperature, and the balloon would rise.

2) You could attempt to increase the pressure of the atmosphere on Earth, thus making the atmosphere gain density…but then this would put pressure on everything, including the hot-air balloon, causing the hot-air balloon to shrink in size and displace less atmosphere = less air contained within = reduced density, so we’d be right back to where we started, right?…….UNLESS we somehow were able to isolate the pressures of the balloon from the outside atmosphere (in a sense, a ‘reverse pressurization’ of what happens on a plane, so instead of sealing to contain higher pressure within, we seal to contain a lower pressure system)… In that case, we’d have two isolated pressure systems, and no matter what temperature the inside of the balloon and the outside atmosphere shared (assuming the buoyancy of the balloon generated from the interaction with the surrounding atmopshere is enough to counteract any gravitational forces), the balloon would always rise.

3) Since #2 isn’t really in the spirt of a hot-air balloon (since those pressure systems would be isolated), let’s think about the ping-pong ball. If that half ping-pong ball had been at the bottom of the ocean with a little bubble under it…it would have been crushed and compressed, the force of the fluid around it (6 miles of water) would have overcome any static force the outside of the ball would have generated against the water…and in a little way, Earth’s atmosphere is the same way when it interacts with a hot-air balloon - if the pressure of the surrounding air increases, the balloon will compress, it doesn’t have much of a ‘defense’ because the material used to trap the hot air within is light and not rigid, this helps with functionality and to reduce weight.

THIS characteristic of the ballon specifically prevents a hot-air balloon from performing what *pingpong*’s original inquiry was about…now, if you could create a large, rigid balloon that trapped air like a half of a ping-pong ball, and didn’t change VOLUME regardless of the pressure surrounding it, then yes, you could have an Earth atmosphere with 85 degrees, 4 bar pressure and 85 degrees, 1 bar pressure atmosphere inside the ‘balloon’, and it would rise.

Make sense? (sorry, I just **** it when people say the right answer and simultaneously think they’re right but, for whatever