Generally, a hydrogen balloon would float when released in air, but what will happen when that hydrogen balloon has a mass of 10kg. Is it the density that decides whether an object floats in air or the mass (inertia)?
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2 Answers
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If the Hindenburg could float (at least until a particular disaster struck) I'm sure a 10kg hydrogen balloon could.
It is indeed relative density that decides whether an object floats or sinks in any particular medium.
Archimedes' principle states that objects experience a buoyancy force equal to the weight of the fluid they displace. This suggests if the object in question is less dense than the liquid, it will displace relatively more fluid than it would need to displace to just stay still, so it will experience a net upwards force.
The opposite happens if the object is denser than the fluid it's submerged in, thus objects denser than their surrounding medium sink.
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A hydrogen balloon floats the same way a boat does, by the volume of the medium it is floating in, that is displaced. If we ask would a 10kg boat float, the answer depends on the volume of water displaced and the relative density of the boat compared to the water. The same goes for the balloon. It will float if it displaces sufficient air, so it depends on the volume of the balloon relative to its mass (i.e. its density relative to air). I think MythBusters managed to get a lead balloon to float in air, by making a sufficiently large helium filled balloon out of very very thin lead foil.
