ElI5: Explain boyancy please


If I have a steel canister of helium and a balloon attached to the valve. If the helium is in the canister, the assembly stays on the ground. If I let the gas into balloon it floats. Why?

There is no change to the amount of energy, the gas is de-pressurised from the canister to the balloon and the density inside the balloon changes, but the overall weight stays the same.q I should know, I just can’t figure out why exactly.

In: Physics

You answered your own question.

>the density inside the balloon changes,

Take a given total *mass* of helium, balloon, and tank.

Use it to fill up the space that used to contain the same *mass* of nitrogen, oxygen, argon, CO2, and other trace gasses that make up atmospheric air.

You now have something that experiences the same force due to gravitational acceleration as the air it’s displacing, and thus becomes able to float in mid air.

Displace another gram of air and your helium assembly will start rising.

I think you’re imagining the tank and balloon as one object, but they aren’t. You open the valve to fill the balloon…*and then close the valve again*. At that point, the balloon is touching the tank, but the stuff inside each of them is separate.

This set up is no different than trying off the balloon and sticking it to the tank with tape. Same idea. In both cases, the balloon wants to float but is affixed to the tank. The tank doesn’t float because the gas inside is compressed so that it’s much more dense than air/the loose gas in the balloon.

The total mass of the balloon + tank + helium is the same both before and after. The volume once the balloon is filled is much larger, so the volume of the system is now larger.

Density is mass / volume. While the mass remains the same the volume increases, making the density smaller. If the density is small enough the tank and balloon will float in air.

You are really asking 2 different questions. Buoyancy is how much water/oil/etc. an object needs to displace, or push to the side to float instead of sinking.

Why the balloon floats when you put helium in it is altogether different.

Air surprisingly enough, has a weight to it. The air we breathe has several different gasses mixed in, that each weigh different amounts. Helium is much higher than the other gasses on the periodic table, meaning each molecule, or the smallest piece of helium, weighs less than every other gas that we breathe. So when you fill the balloon with helium it floats up until it reaches the point in the atmosphere where it floats on top of the other, heavier gasses.

Buoyancy is a force pushing upwards opposite to gravity. The steel canister is being pushed up by the buoyancy of the helium inside, however the gravitation forced pushing it down is greater (because steel is heavy) so it stays on the ground.

When the helium is transferred to a balloon, the buoyancy force is greater than gravity because the balloon is very light and therefore it floats upwards.

The force of buoyancy is dictated by the mass of the object relative to the mass of the volume of material (like air or water) that it displaces. If the object displaced more mass than it itself has, then it moves upwards. In other words, if the overall density (mass divided by volume) of the object is less than the density of the medium (air, water, etc.) it’s in, it’ll float.

It’s all about the density – grams per cubic centimetre.

Try sinking a beach ball in a bathtub.

To push it down below the surface, you have to push a beach-ball sized amount of water *up* to get it out of the way. You can see how the water level rises in the tub – it’s got nowhere else to go.

Water is heavy and does not like being up. It wants to be down, so it keeps pushing back. The beach ball wants to be down, too – but it can only push down with the few grams that a beach ball weighs.

The beach-ball-sized blob of water is vastly heavier, and wins; the beach ball is pushed back out.

Now empty the beach ball and melt it down to a solid plastic marble, and try to sink it again.

Well hey, now you only have to lift up a marble-sized lump of water – and depending on the kind of plastic, it may well have the weight to do that itself.

That’s all that’s going on. You compare the weight of the object with the weight of the *same volume* of water. The object and the water can’t be in the same place, so you have to lift the water up to get it out of the way – and the amount you have to lift is the same size as the object.

If that object only takes a little bit of room, it only has to lift a little bit of water in order to sink.

If the object takes up a lot of room, then it has to lift a lot of water.

Air works exactly the same way. Your helium balloon weighs less that the volume of air it has to push out of the way, while your helium tank weighs a lot more.