i was reading a previously locked thread about the compressed air from tires in the movie transporter three, and the writer asked if compressed air is heavier than regular and therefore does not float. i was just wondering, isn't the biggest proof for this from the way every submarine rises, by letting air into the ballast tanks - where did that air come from when the sub is at the bottom of the sea? clearly, it was compressed and therefore much less buoyant than it would be at atmospheric pressure, and now when released it begins to spread and become more buoyant?

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  • what actually is your question? you talk a lot about what happens but i can't see a question –  Cursed Apr 24 at 14:08

The buoyant force on a submerged object doesn't have anything to do with the density of the object. Basically, the buoyant force is equal to the weight of an equal volume of water.

The net force on a submerged object however is the difference between the buoyant force and the force of gravity. So, if an object is more dense than water, gravity wins and the object sinks, but if the object is less dense than water, buoyancy wins and the object floats.


Air does a pretty good imitation of an ideal gas at atmospheric pressure or, at the pressures that can be achieved by a shop compressor. That means, its density is proportional to its absolute pressure.

When you get up to scuba tank pressures, it becomes less ideal. I don't know exactly how it behaves at those pressures, but it still is the case that increasing the pressure will increase the density.

Part of the reason why a scuba diver wears a buoyancy compensator (basically, a personal version of a submarine's ballast tanks) is because the air tank on the diver's back keeps getting lighter as the diver uses up the air, and so the diver needs to keep letting air out of the B.C., in order to keep from floating to the surface.

Compressed air and 'regular' air weigh the same amount, as it does not physically get heavier. In truth compressed air is regular air. The main difference between the two is that regular air doesnt have a large restricting factor like the air in a tire does. The air in the tire is constricted and cannot expand as easily, as more air enters the tire the pressure in the tire increases, not the weight.

Since when air condenses it has more pressure, it exerts more downward force then non-condensed air. As a result it loses buoyancy and allows the sub to sink. A major factor that effects subs is its mass and its cargo's mass. When an object is more massive the effects of gravity is much more obvious and seemingly amplified. Probably the most over looked factor here is the water. Water has resistance as well and it is possible that the combined with the mass of the sub and cargo it will counteract the buoyant force of the condensed and non-condensed air.

  • "as more air enters the tire the pressure in the tire increases, not the weight" It increases the amount of air, which increases both the pressure and the weight when volume and temperature are constant. "Since when air condenses it has more pressure, it exerts more downward force then non-condensed air" It also has more upwards and sideways forces that should have a net balance, so this is practically irrelevant. This really isn't a good explanation. –  JMac Apr 24 at 14:28

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