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.
Thanks
In: Physics
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.
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