What force causes a balloon to float?

[timothychoi]

When I blow a balloon with CO2, it falls as it is heavier than air. Then it is gravity that causes the balloon to fall.
When I buy a balloon filled with helium gas from a store, it floats as it is lighter than air. But what force is causing the balloon to float?

 

[berkeman]

When I blow a balloon with CO2, it falls as it is heavier than air. Then it is gravity that causes the balloon to fall.
When I buy a balloon filled with helium gas from a store, it floats as it is lighter than air. But what force is causing the balloon to float?

https://en.wikipedia.org/wiki/Buoyancy

 

[timothychoi]

https://en.wikipedia.org/wiki/Buoyancy

I see. It is like an airplane.

 

[berkeman]

I see. It is like an airplane.

No, more like a Blimp...

 

[timothychoi]

No, more like a Blimp...

Oh. I noticed that an airplane floats by "lift". Thanks.

 

[davenn]

Oh. I noticed that an airplane floats by "lift". Thanks.

an aeroplane doesn't float by lift it flies, air must move across the wings ( above and below) to cause lift
If it doesn't keep moving, it will stall and fall from the sky

 

[Cutter Ketch]

When I blow a balloon with CO2, it falls as it is heavier than air. Then it is gravity that causes the balloon to fall.
When I buy a balloon filled with helium gas from a store, it floats as it is lighter than air. But what force is causing the balloon to float?

The force causing the balloon to rise is also gravity! It is not like an airplane. It is more like a bubble under water. Gravity is pushing the water down. As the bubble goes up, that amount of water goes down. So the bubble going up is effectively water falling. Of course gravity is pulling down on the air in the bubble too, but with less force because it is lighter than an equal volume of water. So in net the bubble rising is equivalent to something (the difference in weight between the water and the air) falling.

With your CO2 balloon the rubber+CO2 is heavier than an equal volume of air and that difference in weight falls under gravity. With helium the rubber+helium is lighter than an equal volume of air, and, again, that difference in weight falls under gravity.

That is an energy argument. The way this works in detail is that the weight of the water causes a pressure gradient. The water on top has no weight pressing down on it. Below the surface the water is being pushed down by the weight of all the water above that depth. The deeper you go the greater the weight above and the higher the pressure. That means that the bubble experiences a higher pressure on the bottom of the bubble than on the top. There is a net force upward. It can be shown that the net force upward is equal to the weight of the missing water. This is called buoyancy. If the contents of the bubble weigh less than the missing water, the net force of buoyancy + gravity is upward, and vice versa.