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From my understanding objects do not fall but are pulled to the earth from gravity.

With this in mind, I can't understand why if helium filled balloons are not pulled by gravity then shouldn't they be stationary in the sky (or drift like objects in space with no gravity) rather than actively move away from the earth?

Is gravity pushing it rather than pulling it? Why is this?

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Buoyancy my friend, buoyancy. Look it up. – user346 Dec 31 '10 at 19:49
@space_cadet - I thought buoyancy only had to to with floating in fluid? I am referring to floating in air. – JD Isaacks Dec 31 '10 at 19:53
Buoyancy works in any sufficiently amorphous medium, and in any case "fluid" refers to liquid and gas phases. Air is a gas, and therefore a fluid. – dmckee Dec 31 '10 at 19:56
Yeah, the physics definition of 'fluid' rather differs from the everyday one. It's often used to refer to any thermodynamic medium. – Noldorin Jan 1 '11 at 21:32
up vote 6 down vote accepted

Helium balloons are pulled by gravity, as are all objects with mass. The reason they don't fall is that there is another force acting on them, a buoyant force from air pressure that is equal to the weight of the air displaced by the balloon.

The reason you don't float is that the weight of the air you displace is quite a bit less than your weight (a person is more dense than air). The reason a normal balloon doesn't float is that the weight of the air it displaces is just a little bit less than the weight of the balloon (because it is filled with air, but the rubber of the balloon itself is more dense than the air).

The analogy you want is to objects floating (or suspended) in water. Most rocks sink to the bottom, pulled by gravity, because the weight of the water they displace is less than their own weight. A bowling ball (ironically) is very close to the same density as water, so it will float suspended in mid-water, just like the helium balloon that has leaked a little bit.

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Just a tad about how bouyancy works. Any fluid in a gravitational field possesses a pressure gradient, (which if the gas/liquid is in equilibrium) counterbalances the effect of gravity. Gravity acting on such a fluid creates this pressure, which is referred to a hydrostatic pressure. To make a long story short, the external pressure (of the air) is greater at the bottom of your ballon than at the top. So the net external pressure on the ballon is imbalanced, providing a net upward force equal to the weight of the displaced air.

If you put your ballon in space (I'll ignore the observation it would pop), there is no pressure, and therefore no imbalance of pressure to create the bouyancy effect.

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+1 for the pressure gradient which is quite important – Leos Ondra May 30 '13 at 8:30
@Omega Centauri Very nice explanation. A gedanken question for you: A large volume but low height vessel of low density would displace a significant amount of the surrounding fluid, but would be subject to a negligible pressure gradient across it's height; would we expect a balloon such as this to rise? If so, why? – James Pattarini Feb 11 at 10:44

protected by Qmechanic Sep 8 '13 at 14:59

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