surajit001
Topic Author
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Joined: Wed Dec 19, 2012 1:09 pm

### Difference Between Lift Force And Buoyant Force

I really want to know the difference between lift force and buoyant force? When does a fluid exert lift force on a body and when does it exert buoyant force. In my opinion, buoyant force occurs when the fluid is at rest and when we immerse an object in the fluid, the fluid exerts a buoyant force. Whereas lift force occurs in a moving fluid and the pressure difference on the object creates the lift force. Can someone kindly explain me the basic difference between these two forces?

PapaChuck
Posts: 111
Joined: Mon Aug 16, 2010 2:51 pm

### RE: Difference Between Lift Force And Buoyant Force

A helium balloon is positively buoyant. Helium is less dense at a given pressure than air, so there is a buoyant lifting force. It's the same idea as floating a cork in water.

An airplane is much more dense than air. An aerodynamic lifting force is therefore needed to lift it off the ground.

PC
In-trail spacing is a team effort.

tdscanuck
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### RE: Difference Between Lift Force And Buoyant Force

 Quoting surajit001 (Thread starter): I really want to know the difference between lift force and buoyant force?

Lift is due to motion through a fluid. Buoyancy is due to static pressure gradients within a fluid.

 Quoting surajit001 (Thread starter):When does a fluid exert lift force on a body and when does it exert buoyant force

Fluids will exert lift forces any time the body is moving (the net sum may be zero, like a symmetric body at zero AoA).
Fluids will exert a buoyant force any time there is a static pressure gradient in the fluid (usually caused by gravity, but not always).

 Quoting surajit001 (Thread starter): In my opinion, buoyant force occurs when the fluid is at rest and when we immerse an object in the fluid, the fluid exerts a buoyant force.

Just immersing a body in a fluid doesn't cause a buoyant force. Buoyancy is really just a pressure differential, so it only happens when you have a pressure gradient. If you stick a cork in water in zero-g (e.g. on the Space Shuttle) it doesn't experience a buoyant force because there's no pressure gradient.

 Quoting surajit001 (Thread starter):Whereas lift force occurs in a moving fluid and the pressure difference on the object creates the lift force.

Yes. Both forces are caused by pressure differential. The difference is why there's a pressure differential...for buoyance, the differential was in the fluid whether the body was there or not. For lift, the differential is caused by the moving body.

 Quoting PapaChuck (Reply 1):A helium balloon is positively buoyant. Helium is less dense at a given pressure than air, so there is a buoyant lifting force.

Careful about "buoyant lifting"...that just means the buoyant force is greater than the weight. All balloons experience a buoyant force, regardless of the density of the gas inside them. They only float if the buoyant force exceeds their weight.

 Quoting PapaChuck (Reply 1):An airplane is much more dense than air. An aerodynamic lifting force is therefore needed to lift it off the ground.

Airplane still experience buoyancy (exactly the same amount as a blimp of equivalent volume). But, as you note, airplanes are so dense relative to air that the buoyant force on them is effectively nil so it's almost always ignored and you just deal with lift. Note that that same airplane is *not* very dense relative to water, which is why airplanes generally float pretty well as long as they don't have holes in them.

Tom.

A320ajm
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### RE: Difference Between Lift Force And Buoyant Force

Buoyancy is only dependant on lateral density variations too. Lift is caused by variations in pressure above and below a wing.

A320ajm
If the British Empire and its Commonwealth last for a thousand years, men will still say, 'This was their finest hour.'

jetmech
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### RE: Difference Between Lift Force And Buoyant Force

 Quoting tdscanuck (Reply 2):Buoyancy is due to static pressure gradients within a fluid

Interesting. I always had buoyancy figured out as being due to the density difference between the two entities in question. My mental picture was that the denser fluid experienced a greater force from gravity for a given unit volume. This "pulled" the denser fluid "down" around the less dense fluid causing it to rise in response.

 Quoting tdscanuck (Reply 2): If you stick a cork in water in zero-g (e.g. on the Space Shuttle) it doesn't experience a buoyant force because there's no pressure gradient.

Could the absence of a net gravity force also explain this behaviour?

I'm not questioning the pressure gradient explanation (which is also given by Wikipedia); I'm just surprised that gravity acts indirectly via the production of a pressure gradient rather than more directly as I first imagined.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair .

tdscanuck
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Joined: Wed Jan 11, 2006 7:25 am

### RE: Difference Between Lift Force And Buoyant Force

 Quoting jetmech (Reply 4):I always had buoyancy figured out as being due to the density difference between the two entities in question.

This is a mixup between the buoyant force (which depends only on the object's volume and the fluid's density) and the net force (buoyancy minus weight). It's only the net force that depends on the density of the object (and hence the density difference) because the object's density determines its weight.

 Quoting jetmech (Reply 4):My mental picture was that the denser fluid experienced a greater force from gravity for a given unit volume. This "pulled" the denser fluid "down" around the less dense fluid causing it to rise in response.

I used to have this mental picture too and it's intuitively appealing. It's really an energy argument...having the heavier stuff on the bottom (whether that's the fluid or the object) is a lower energy state and hence favoured. The danger with thinking about it in terms of density difference giving rise to the buoyant force is that the gravity force on both the fluid and the object is the same whether the object is in the fluid or out of it.

Part of the problem with the buoyant force is that you can easily move the math around to get equations that work in terms of density differences and, since it's just algebraic manipulation, the answer is still valid. However, it obscures the physics behind the math. Most of the time, working in density is fine as long as you never forget the assumptions behind the mathematical manipulations that got you there (constant uniform densities, no acceleration, etc.). However, it's not as fundamental on the physics side and I find it more accurate, though sometimes less intuitive, to work in pressure.

Tom.

jetmech
Posts: 2317
Joined: Wed Mar 29, 2006 2:14 am

### RE: Difference Between Lift Force And Buoyant Force

 Quoting tdscanuck (Reply 5):It's only the net force that depends on the density of the object (and hence the density difference) because the object's density determines its weight.

Fair enough. My mental picture was developed around the case of a gas such as air interacting with water. In this case, the reduction of the buoyancy force due to weight would not be too significant. An object like a ship would of course would be different.

 Quoting tdscanuck (Reply 5):I used to have this mental picture too and it's intuitively appealing. It's really an energy argument...having the heavier stuff on the bottom (whether that's the fluid or the object) is a lower energy state and hence favoured.

It is interesting how nature seems to always seek the state of least energy. It would be a pretty fundamental, almost philosophical exercise to understand why. Perhaps this is some sort of explanation for the lack of a buoyancy force whilst in space?

Ignoring an electro-magnetic interaction and anything more exotic also, nature would only have gravity as a tool to exert influence over a fluid and an object of a different density within that fluid. In the situation of zero net gravity nature cannot influence the fluid / object system, hence that lack of preference for any particular physical arrangement between the two entities?

 Quoting tdscanuck (Reply 5): However, it's not as fundamental on the physics side and I find it more accurate, though sometimes less intuitive, to work in pressure.

I always seek the fundamental explanation, even if it may be beyond my intuitive grasp   .

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair .

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

### RE: Difference Between Lift Force And Buoyant Force

 Quoting jetmech (Reply 6):It is interesting how nature seems to always seek the state of least energy. It would be a pretty fundamental, almost philosophical exercise to understand why.

If you get to playing with Lagrangian mechanics, there's basically a one-to-one correspondance with energy gradients and force gradients...the forces in a system tend to push it towards the lower energy state.

 Quoting jetmech (Reply 6):Perhaps this is some sort of explanation for the lack of a buoyancy force whilst in space?

Basically, yes. There's no preferred energy arrangement.

 Quoting jetmech (Reply 6):In the situation of zero net gravity nature cannot influence the fluid / object system, hence that lack of preference for any particular physical arrangement between the two entities?

Exactly.

Tom.

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