What makes a hot air balloon go up?

Question

Hot air balloons work by using heat to make the air inside the balloon lighter than the air outside the balloon is what I heard. In other words, HOT AIR. But that sounds like baloney. If that is true then what the hell is holding you G@G clowns down? I guess it's a good way to break up with someone. If you can get them to talk politics all that hot air will carry them away. Your theories on all this?

Massageman · Accepted Answer

Answer 1:
Hot air rises because when you heat air (or any other gas for that matter), it expands. When the air expands, it becomes less dense than the air around it. The less dense hot air then floats in the more dense cold air much like wood floats on water because wood is less dense than water. This floating effect in a less dense medium is called a buoyant force or a displacement force.

We can model what I just said with a little physics. Consider the air to be an ideal gas (this is a good approximation which neglects the interaction of air molecules with each other).

The pressure P, volume V, amount of gas N, and temperature T can all be related by the ideal gas equation as
P*V=N*R*T
where R is the ideal gas constant (8.314 in units of J/(mol*K)).

Say you start with an amount of air N at pressure P, volume V1, and Temperature T1 and heat it in a balloon (we will neglect the mass and elastic properties of the balloon for simplicity) to temperature T2.

The ideal gas equation can be rewritten as
P*V1/(N*T1) =R=P*V2/(N*T2)
which with a little algebra can be solved to give
V2=V1*T2/T1 .

From this we see that if you heat a gas such that
T2>T1
you get
V2>V1
meaning that the gas expands. Now since you have the same amount of gas, the mass does not change so the density before heating is
d1=m/V1
and the density after heating is
d2=m/V2.

Now the force of gravity on the hot air (Fg) is given by the mass of the hot air times the gravitational constant g (9.8 m/s2)
(Fg=d2*V2*g).
Here the mass of the hot air is given by the density of the hot air times the volume of the hot air. The buoyant force on the balloon is given by the mass of the air displaced times the gravitational constant (Fb=d1*V2*g).
Here the mass of the air displace is given by the density of the surrounding cool air times the volume of the balloon (hot air). Now the buoyant force is up and the gravitational force is down so the net force is given by
F=Fb- Fg=d1*V2*g- d2*V2*g=m*g* (V2/V1- V2/V2)
which can be rewritten using the relation from the ideal gas law (V2=V1*T2/T 1) to give:
F=m*g*(T2/T1-1).

This equation shows that as you heat the air in the balloon T2>T1 (i. e. T2/T1>1) you get a positive force (F>0) which means that the force is up and the balloon rises.

In summary, hot air rises because it is less dense than the surrounding air.

Answer 2:
Hot air pushes out on the air around it, and thus becomes less dense. Less dense air rises because the denser, cooler air sinks because it is heavier and gravity pulls on it more strongly.

Likes2drive · Accepted Answer

I know exactly how it works for hot air balloons but I don’t know how it works for people on here, maybe they are just held down from all the lead up their ass, lol

MORPHEUS_12 · Answer

Gravity is what holds us on the Earth. Example have you every been ridden on a rollercoaster and when you go down the hill you feel the Gs pulling your body down. If you are a 200-pound person and the ride pulls 3 Gs then your body will feel like you was 600 pounds.

Hot air balloons take flight by using Propane. I have been up and with was an awesome experience. The pilot is not constantly wailing on the Propane the whole trip. It's used to heat the air in the balloon. The balloon is solely dependent on air currents to travel. Several years after my flight I went to a small local balloon festival and I watched three people burn up after a balloon hit a powerline. Since I had been up I spotted that they balloon was on fire. I started taking pictures and as the fire got brighter the hair stood up on the back of my neck. I instantly stopped filming. My picture was synicated all over the world.

SnowedIn · Answer

You are right as always. The difference between the GaG clowns and the balloons is that the GaG people breathe so the hot air escapes and ends up here. Balloons trap it in.

nolabels · Answer

The expulsion of hot air here on G@G is what keeps me grounded.

If I stay away from G@G for more than a day, my ass starts floating up in the air.

I have to tie myself down sometimes to keep from drifting away.

If I fart, that usually helps, too. 😊👍

Surely · Answer

If that were the case. Washington DC should have been up up and away many years ago!

exitseven · Answer

When things heat up they expand. When a body expands it displaces more volume but it's weight stays the same. It's buoyancy increase. If it is surrounded by something that is heavier per unit volume , it will float.

Sparkie460 · Answer

They are exhausting the hot air on here. If they didn't release it they might float up in the air. This site is keeping people grounded

MementoMori777 · Answer

I don't think its co2 or oxygen , doesn't it burn like helium or something?

NYCQuestions1976 · Answer

I'm way too D. F. for your theory to work on me.

Anonymous · Answer

The hot air? In all seriousness, I don't know the science behind it.

Guardian45 · Answer

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What makes a hot air balloon go up?

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