- #1
Amaterasu21
- 64
- 17
Hi all,
I understand where Archimedes' Principle comes from in liquids:
If we imagine a cylinder immersed in a liquid of density ρ whose cross-sectional area is A and whose top is at depth h1 and whose bottom is at depth h2:
Force(top of cylinder) FT = ρgh1A
Force(bottom of cylinder) FB = ρgh2A
Buoyant force = ρgA(h2-h1)
= ρA(h2-h1)g = ρVcylinderg = mdisplaced liquidg = weight of displaced liquid
therefore buoyant force = weight of displaced liquid.
I also understand that this works even if the object immersed in liquid isn't a cylinder because pressure at any depth is constant, so the horizontal components of pressure on any surface of the object cancel out, and the vertical components produce the same buoyant force as the cylinder.
But what I don't understand is why this is true for gases as well. I've been told that the buoyant force in gases is equal to the weight of displaced gas. However, gases unlike liquids are compressible and a large mass of gas in a gravitational field (e.g. the atmosphere) will be far denser at the bottom than at the top. Therefore while ρ is constant during the derivation for liquids, it would certainly NOT be constant across the height of the cylinder in a gas!
Force(top of cylinder) FT = ρ1gh1A
Force(bottom of cylinder) FB = ρ2gh2A
Buoyant force = gA(ρ2h2 - ρ1h1)
...and I don't see how we get from here to "= weight of displaced gas!"
Any help would be appreciated!
I understand where Archimedes' Principle comes from in liquids:
If we imagine a cylinder immersed in a liquid of density ρ whose cross-sectional area is A and whose top is at depth h1 and whose bottom is at depth h2:
Force(top of cylinder) FT = ρgh1A
Force(bottom of cylinder) FB = ρgh2A
Buoyant force = ρgA(h2-h1)
= ρA(h2-h1)g = ρVcylinderg = mdisplaced liquidg = weight of displaced liquid
therefore buoyant force = weight of displaced liquid.
I also understand that this works even if the object immersed in liquid isn't a cylinder because pressure at any depth is constant, so the horizontal components of pressure on any surface of the object cancel out, and the vertical components produce the same buoyant force as the cylinder.
But what I don't understand is why this is true for gases as well. I've been told that the buoyant force in gases is equal to the weight of displaced gas. However, gases unlike liquids are compressible and a large mass of gas in a gravitational field (e.g. the atmosphere) will be far denser at the bottom than at the top. Therefore while ρ is constant during the derivation for liquids, it would certainly NOT be constant across the height of the cylinder in a gas!
Force(top of cylinder) FT = ρ1gh1A
Force(bottom of cylinder) FB = ρ2gh2A
Buoyant force = gA(ρ2h2 - ρ1h1)
...and I don't see how we get from here to "= weight of displaced gas!"
Any help would be appreciated!