Alright, pressure is acting all upon us, from all directions. But say you are at 5 meters floating on the ground, for the sake of argument, since we're ignoring gravity here anyway. Wouldn't the hundreds of kilometers of atmosphere above you overcome the pressure pushing you from below, negating any buoyant effects, even from objects lighter than air? Enlighten me in this question.

The explanation of this fact was discovered by Archimedes of Syracuse, Sicily, more than two centuries b.C., and is so simple even a child can understand it. It works both for liquids (e.g. water) and gases (e.g. air).

The figure is explicative enough. Anyway, let's begin with some facts. Pressure acts in all directions. Pressure depends by the height of the column of air or water above us (scuba divers know that if they are twenty meters deep the pressure is double respect if they are ten meters deep).

Let's say for sake of debate that the atmosphere is hundred kilometers high. There is an object buoyant in the air. Let's say it's one meter tall. Well then, on top of it (from above to below) there is the pressure of hundred kilometers of air. On bottom of it (from below to above) there is the pressure of hundred kilometers of air PLUS ONE METER. This is the key concept! The pressure from below is bigger than the one from above, due the dimension of the object, and this principle is universal, it works for every object immersed in a fluid.

If we want to say it more exactly, let's say that the force exerted by the fluid from below to above is equal to the weight of the fluid removed by the object (e.g., if the object is one cubic meter it removes one cubic meter of air or water, depending where it is immersed).

The case of the helium balloon. Let's say it is one cubic meter in volume, that is it removes one cubic meter of air. One cubic meter of air weighs

*about* one kilogram, and then the balloon get a force from below to above equal to a kilogram. But the helium is lighter than air, and the entire balloon (the wrapping and the helium inside) weighs say half a kilogram.

And now let's put it all together.

Gravity attracts the balloon with a force of half a kilogram (because the balloon weighs half a kilogram); the atmosphere pushes the balloon from below to above with a force of a kilogram (because it removes one cubic meter of air that weighs one kilogram).

Net result: the balloon get pushed from below with a force of half a kilogram, and it goes up.

If the object has a density bigger than air... well, i strongly hope you can elaborate it yourself.

Let me add that believing gravity doesn't exist and we stay on the ground due to the atmospheric pressure is really a shame.

Bye.