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Name: Casper
Status: student
Grade: other
Country: Denmark
Date: N/A 


Question:
Hello, Maybe this is a stupid question, but I have never received a satisfying answer, so I will try my luck here, worst case scenario is being called stupid, so why not. My question is: Is it theoretical possible to create a container that can handle the pressure of a high 'enough' level of vacuum to become zero-weight or even float like Helium? Or how close is it possible to get to the zero weight of the container. My thought is based on Helium balloons, and my thought is that a high level of vacuum has to be better than the expensive Helium.



Replies:
Hi Casper,

The key principle to consider is "buoyancy" -- helium balloons float in air, and air has a particular density (that depends on temperature and pressure). If you build a container whose average density is less than that of air (in other words, its mass is lower than the mass of an equal volume of air), the container will float. You can also create buoyant lift by simply heating air (such as with a hot air balloon). Hotter air expands, and therefore has lower density, than colder air.

Your first question about 'is it possible' to build a lighter-than-air craft, I would have to say 'yes', although I am not aware of such a device actually having been built using vacuum rather than a lighter-than-air gas such as hydrogen or helium. I would add that its weight cannot be zero, though, as the container must have some weight (blimps are lighter than air, but are still very heavy in terms of actual weight - they just have a very large volume). The reason the vacuum route may not have been pursued I would guess is because of your second question. Cost is an interesting twist. It is very expensive to build a strong, rigid container that is also well-sealed enough to hold vacuum, and much easier to build, basically, a big sack that you can fill with a lighter-than-air gas. If your goal is to be cost-effective, the "big sack of gas" is a pretty cheap option.

Hope this helps,
Burr Zimmerman


Casper,

That is not a stupid question, that is a great question. You question is about Archemedes' Principle, where the evacuated container displaces air and produces a buoyancy force equal to the weight of the displaced air. The theory here is sound. If you could produce a lightweight container which can withstand the pressure without collapsing then this would work. And in fact this idea was proposed in 1670 by Father Francesco de Lana, an Italian Jesuit priest, who suggested using very thin copper to make lightweight evacuated spheres. Such spheres would collapse under the pressure, however. Even today we do not have any materials strong enough to withstand such pressure while remaining light enough so that the buoyancy force exceeds the gravitational force. You would need to fill the interior of such an object with another gas to provide the outward pressure necessary to resist the inward pressure of the atmosphere. Hydrogen weighs only 1/10th that of air and is the ideal candidate although it does have safety concerns (it is highly flammable).

John Strong


Be assured, neither you nor your question is "stupid". Many have asked this question, and confusion takes hold. I hope I do a little bit clearer.

The principle of buoyancy is this: An object is exposed to a buoyant force that is equal to the WEIGHT of the VOLUME of the fluid that the object displaces when placed on/in the liquid. Now this requires some thought because "what is equal" is the weight (force) that weighs the same amount as the same amount of fluid (volume) displaced. That is a little tricky. Possibly an example will help.

Suppose you float an aluminum "boat" on water. If the aluminum is crumpled into a ball, the "boat" will sink because you have decreased its volume. But, if the aluminum foil is shaped into a "boat" so that it occupies a larger volume, the "boat" will float. If you have a closed boat, it does not matter what is in the boat, what the "boat" is made of -- only its shape. What is important is the weight of the volume of water that is occupied by the "boat".

If you could make a balloon, the volume whose weight exceeds the volume of air displaced by the surrounding air, it will sink. If the balloon's weight is less than the weight than the amount of fluid displaced, it will float. So, properly designed, the "boat" could be lead or concrete. The important issue is, "The weight of the volume of fluid displaced by the object.

Vince Calder


Hi Sophia,

This is a reasonable question. If it were possible to construct a lightweight and strong container and remove the air from it, the container would float like a balloon does. That is true. Unfortunately, the density of air is quite small, so that the weight of the container would need to be very small for it to float. Because the pressure of air is fairly high, there are no practicable materials that are light and strong enough to withstand the pressure and still float. That is why it is necessary to use an internal gas (helium, hydrogen) to support the pressure to make the container (blimp or balloon) practicable.

This is different from floating in water. Water is dense, and has large but manageable pressure for containers in the ocean (like a submarine). Standard steel construction is adequate to make a vessel that is buoyant in water in reasonable depths.

Robert Erck



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