For the technological education course I have to give a talk about the hot air balloon with my girlfriend. The most important part of the talk is how the hot air balloon takes off. The teacher says we need to bring in Archimedes’ law. But what is this law and what does it have to do with how a hot air balloon takes off?
Answer
You have to combine different things with each other.
Air has a mass and is therefore attracted to the earth. So the air pushes on the earth, this is what measuring when we determine the air pressure. This pressure, of course, depends on how much air is above us: if you look at a stack of books, the pressure force at the bottom is also greater than at the top.
If you now place an object with a certain volume in air, the compressive force on the underside will be greater than the compressive force on the top: the volume experiences a net force upwards: this is the Archimedes force. It can be shown that this Archimedes force is equal to the weight of the displaced air.
On the other hand, the object also has its own mass and experiences a downward force due to the gravitational pull of the earth. Whether the object will rise, float or fall depends on which of those two forces is greater.
Air expands as it heats up. As a result, the density will decrease. If you fill a sufficiently light bag with enough warm air, the whole will become lighter than air and float.
Answered by
Prof Walter Lauriks
Physics Acoustics
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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