Need a physicist to explain fluids to me

[VivaTequila]

What determines whether something floats or not? We happen to be studying this at uni, and it's got me thinking a lot.

If you blow up a balloon with air from your lungs and let it go, it all rushes out because there is a pressure different between the balloon and the atmosphere. The balloon is at a higher pressure than the atmosphere, so the particles inside want to reach equilibrium (or rather as they bounce around willy nilly, it just so happens that this is the effect due to entropy). I hope you then agree with me that the gases inside the balloon are at a higher pressure than the surroundings, such that the force exerted on the walls of the balloon is great (P = F / A where A is a constant, but the F increases, so P increases with more gas particles put into the same space)

So why is it that a hot air balloon floats? A hot air balloon simply uses hot air to achieve the same means - the heat is translated into kinetic energy of the particles, which then have a greater speed, and they exert a greater force on the walls of the balloon, no?

So how does floating in fluids work?

[xZero]

Hot air balloon floats because of buoyancy, heating the air inside the balloon will make it lighter than surrounding air, causing it to float.

What determines whether something floats or not?

Quite simply, if lift is equal or greater than weight then it floats. In air, lift is generally generated from pressure difference (plane) and in water its buoyancy.